Analog FM Mono receivers versus FM HD

[Black_Shire]

Hello All,

When I arrived at work yesterday, I found that one of my co-workers had bought a new "boom-box" to replace our failing old one in the office. The new unit is a Casio CD-312S, which is a CD/Cassette/AM receiver/FM receiver.

I noted with interest that the radio section has three selections: AM, FM Mono, and FM Stereo. My co-worker who bought it was off from work yesterday, but looking it up online after I returned home, I found that the CD-312S has a Manufacturer's Suggested Retail Price of $56.00.

What interests me (in connection with IBOC) is the fact that this "boom-box" has an FM Mono option. This is obviously intended to allow clear reception of distant or weak FM signals when they would otherwise be marred by static if the listener tried to receive them using the FM Stereo mode. Casio wouldn't have offered the FM Mono option if they didn't think people would listen to programming in mono. The fact that people will listen to FM in mono if the content is compelling is yet another sign that IBOC is superfluous at best.


-- Black Shire

 

[SUPERCASTER]

As with all media, such as the internet, content is king. Technology, the enabler.
If you are really interested in hearing something, mono may suffice. The mono FM signal can have an over 20 db noise and multipath advantage over a noisy stereo signal, and double the range. An FM station transmitting with iBiquity digital HD is at a further disadvantage because the digital HD hash interferes with, and further reduces, the analog stereo signal to noise ratio. Without HD, that digital noise would not be there. Hence, the HD digital signal's introduced noise, which is added to the analog FM stereo signal, seems to disappear when the HD kicks in. That mono switch is even more important until the HD FM stations discover the error of their ways and stop the HD buzz.
The myth that digital HD is "immune to multipath" is just a fabrication to sell expensive, new, HD radios.
iBiquity HD digital just mutes, plays the buffered few seconds of audio, and/or rolls back to analog in the presence of multipath. Just the same as your digital cell phone.
The manual FM mono switch is needed now, more then ever, with the introduction of HD digital hash to the analog stereo signal.

 

[700WLW]

Hello All,

When I arrived at work yesterday, I found that one of my co-workers had bought a new "boom-box" to replace our failing old one in the office. The new unit is a Casio CD-312S, which is a CD/Cassette/AM receiver/FM receiver.

I noted with interest that the radio section has three selections: AM, FM Mono, and FM Stereo. My co-worker who bought it was off from work yesterday, but looking it up online after I returned home, I found that the CD-312S has a Manufacturer's Suggested Retail Price of $56.00.

What interests me (in connection with IBOC) is the fact that this "boom-box" has an FM Mono option. This is obviously intended to allow clear reception of distant or weak FM signals when they would otherwise be marred by static if the listener tried to receive them using the FM Stereo mode. Casio wouldn't have offered the FM Mono option if they didn't think people would listen to programming in mono. The fact that people will listen to FM in mono if the content is compelling is yet another sign that IBOC is superfluous at best.


-- Black Shire

My RS PLL AM/FM radio has a DX/LOC switch, that is supposed to work only for FM - how does an FM mono switch differ from a DX/LOC switch and/or a Wide/Narrow bandwidth filter ?

 

[SUPERCASTER]

Hello All,

When I arrived at work yesterday, I found that one of my co-workers had bought a new "boom-box" to replace our failing old one in the office. The new unit is a Casio CD-312S, which is a CD/Cassette/AM receiver/FM receiver.

I noted with interest that the radio section has three selections: AM, FM Mono, and FM Stereo. My co-worker who bought it was off from work yesterday, but looking it up online after I returned home, I found that the CD-312S has a Manufacturer's Suggested Retail Price of $56.00.

What interests me (in connection with IBOC) is the fact that this "boom-box" has an FM Mono option. This is obviously intended to allow clear reception of distant or weak FM signals when they would otherwise be marred by static if the listener tried to receive them using the FM Stereo mode. Casio wouldn't have offered the FM Mono option if they didn't think people would listen to programming in mono. The fact that people will listen to FM in mono if the content is compelling is yet another sign that IBOC is superfluous at best.


-- Black Shire

My RS PLL AM/FM radio has a DX/LOC switch, that is supposed to work only for FM - how does an FM mono switch differ from a DX/LOC switch and/or a Wide/Narrow bandwidth filter ?

The FM mono switch manually turns off the stereo detection circuit, thus improving signal to noise ratio and sensitivity at the expense of the stereo separation. If automatic stereo/mono switching is all that is available, then a marginal analog signal, severe multipath, or digital HD interference, may cause the radio to annoyingly switch back and forth between mono and stereo. Mono analog mode also gives more immunity from the new, added digital HD radio hiss, buzz, and interference.
On FM your DX/local switch changes sensitivity, and wide/narrow switch narrows bandwidth and increases selectivity, decreasing some noise, but sometimes at the expense of distortion or fidelity.

 

[700WLW]

Thanks !

 

[radioskeptic]

First you have to understand how analog stereo FM works.

Back in the late 1950’s, the FCC made it clear that any FM system they adopted would be compatible with existing mono receivers, just as they had required that color TV be compatible with monochrome reception on existing black-and-white TV’s. All the major proposals met that requirement. Here’s how the Zenith “pilot tone” system does it:

The FM radio signal carries two audio signals, one as straight audio (up to 15 kc) and the other on a double sideband suppressed carrier (modified AM, i.e., without a carrier). (It also carries a 19-kc pilot tone that allows receivers both to decode stereo and switch automatically to mono in the absence of the pilot.)

But those signals are NOT the left and right channels of the stereo program. Instead, they are sum (L+R) and difference (L-R) signals formed by combining the two channels both in- and out-of-phase. Mono receivers use the mono sum signal and ignore the difference. A stereo receiver generates an exact replacement 38-kc carrier for the difference signal by using the same 19-kc that formed the original (suppressed) subcarrier at the station.

The reconstructed 38-kc hi-fi AM difference signal (occupying 23-53 kc) is then fed to two different, out-of-phase AM detectors, whose outputs are combined with two separate but identical audio signals from the main channel FM detector. Here’s the math:

(L+R) + (L-R) = L+R+L-R = 2L, yielding the left channel (right channel cancelled)
(L+R) – (L-R) = L+R-L+R = 2R, yielding the right channel (left channel cancelled)

Now here’s why stereo is innately noisier: FM has a triangular noise spectrum, not a rectangular noise spectrum like AM. That is, the background noise rises in inverse proportion to frequency. So the background noise on those AM stereo difference sidebands is significantly higher than that on the main channel mono audio. On a strong local signal, the audible difference in noise between stereo and mono is negligible, but the difference between the stereo and mono signal-to-noise (S/N) ratios rises quickly as the RF signal level decreases, and even faster as the relative level of adjacent channel interference increases. And remember, I-BUZZ is just another form of first-adjacent interference!

A “DX-LOC” (distant-local) switch reduces sensitivity so that strong local signals won’t overload the RF and IF circuits. A bandwidth switch only changes the overall bandwidth of the receiver’s IF circuitry. The latter will reduce noise caused from adjacent channel inference, but not noise resulting from a signal too weak to give good “quieting” in the ultrasonic range of the stereo difference signal. And narrowing the bandwidth too much will result in significant increase in distortion of one-sided stereo material (though only a slight increase in distortion of a mono signal, or a negligible increase in distortion in the mono component of a stereo signal).

But a mono switch not only reduces noise caused by signal weakness; it also reduces noise from adjacent channels (including a station’s own I-BUZZ) , since most of that noise will affect primarily the stereo component of the stereo signal.

Now here’s the best part: You don’t have to switch to mono in the tuner to reduce noise. If you have a separate amplifier with a mono switch, switching the amp to mono will eliminate most --and if the channel balance is good, virtually all – the noise, since the detected difference signal consists of not just L-R, but of L-R+N, with N being noise. Here’s the math for that:

(2L+N) + (2R-N) = 2L + 2R + N – N = 2L + 2R = mono with the noise cancelled!

 

[hipporadio]

My RS PLL AM/FM radio has a DX/LOC switch, that is supposed to work only for FM - how does an FM mono switch differ from a DX/LOC switch and/or a Wide/Narrow bandwidth filter ?

Obviously the very capable Mr. Wells is getting a head-start on his holiday shopping today, so I’ll attempt to provide a “budget explanation”.

In general a LOCAL/DX switch, when it appears on a lower-priced consumer product, is an indication of a less-sophisticated tuner section. It may affect any one band, or both, depending on the design. It’s an inline RF attenuator designed to knock down signal levels before they enter the receiver’s front-end RF amplifier to lessen the effects of signal overload (i.e.--a strong station appearing numerous places throughout the band nuking weaker ones). Unlike its inclusion on higher-performing and more expensive general-coverage DX receivers (like an AM/SWL/Ham rig)--it never is a “cure-all” as lower-strength signals are simultaneously reduced effectively resulting in a “zero-sum” improvement.

STEREO/MONO mode selection has been around since the advent of FM-Stereo, and was considered essential on most receivers due to the technical nature of the FM multiplex system itself. They have steadily disappeared from lower-priced FM radios over the years, I suspect as merely a cost-cutting measure. So why the switch?

FM stereo is basically “mathematics in motion”. ALL FM stations are MONO, and compatible L+R audio resides within the 50-15kHz spectrum indicative of its fundamental audio frequency range. Stereo FM stations “generate” two additional sub-carriers above that spectrum to enable dual-channel reception. A device in the FM audio chain detects the left-minus-right or “difference” audio and modulates it at a much lower level as a subcarrier centered at 38 kHz. Upon reception, the low level “difference” information is demodulated and mathematically reshuffled back into two equal-level channels of audio. Because the transmitted “difference” is much lower than the main channel “sum”, the receiver needs to make up the difference--thus raising the noise floor significantly in a low signal strength area. Hence, the need for a mode switch to provide for lower noise mono reception should that become necessary.

WIDE/NARROW relates to audio frequency or IF bandpass on AM--and RF frequency “width” (with respect to modulation band-sample) on FM. Both effectively reduce adjacent-channel interference with differing trade-offs. The audio frequency response is changed on AM... Tuner selectivity in regards to the recovered frequency deviation is modified (but not the frequency response of the audio) on FM.

I doubt that the inclusion of a “mono” mode switch on an inexpensive Casio boom-box has much to do with IBOC (or so their intention)... But low signal strength analog stereo reception certainly can suffer from adjacent-channel IBOC carriers--and could be improved by switching the affected radio to mono--as would a NARROW bandwidth selection on FM--reducing detection of the IBOC carriers at the expense of much higher demodulation or audio distortion. Very few current FM receivers provide for bandwidth selection... That's usually a high-priced specialty tuner feature.

OK techies... How did I do on that one?

UPDATE... Radioskeptic has a great "textbook" description... I just caught his post right I made mine!

 

[SUPERCASTER]

From hipporadio:

I doubt that the inclusion of a “mono” mode switch on an inexpensive Casio boom-box has anything to do with IBOC...

As you mentioned, the mono mode switch has been around as long as stereo FM. Automatic stereo/mono switching is somewhat newer.
No, the switch was not recently added to analog radios as a response to IBOC. But if you have a mono/stereo switch, sometimes it can be helpful in reducing HD digital interference. I believe that is what my statement said. Not that it was recently added specifically because of, or in response to, HD radio.

 

[Black_Shire]

I just checked the radios in my house, and my Sangean DT-200V pocket receiver and Emerson CD/AM/FM "boom-box" kitchen counter top radio also have FM Mono switches.

I've used the mono feature on the DT-200V to clearly pick up KYSC 96.9, whose Stereo signal drops out intermittently in some spots in the house.


-- Black Shire

 

[hipporadio]

I just checked the radios in my house, and my Sangean DT-200V pocket receiver and Emerson CD/AM/FM "boom-box" kitchen counter top radio also have FM Mono switches.

I've used the mono feature on the DT-200V to clearly pick up KYSC 96.9, whose Stereo signal drops out intermittently in some spots in the house.


-- Black Shire

The Sangean DT-200V pocket radio is a best of class... I've had one for a couple years, and have never owned a "walkman-style" radio that comes close to it in both audio and reception. The FM rocks--very good AM sensitivity also. C Crane just reduced the price on them to $54.95 to make way for a new Sangean model. 'Don't know if the DT-200V is going away--I hope not--but git 'em while they're hot

 

[700WLW]

Some very knowledgeable people ! I knew that the DX/LOC switch was an attenuator (but, didn't realize it was mainly on cheaper models, such as mine), and that the narrow/wide filter cut adjacent-channel noise, at the expense of the audio quality. On my now returned S350 , there was an RF-Gain knob, that I presumed acted as an adjustable DX/LOC switch - is that somewhat correct ? It also had low-pass filter and wide/narrow switches, but those seemed to have negligible affect; would those be included in only analog-tuned and not PLL tuned receivers ? Some people have complained about PLL noise, but with my little RS radio, it isn't noticeable. Also, how can Grundig claim (and, maybe with the SRIII and CCRadio) that these are high-performance receivers, when my $25 RS and car radios were able to get the same DX stations ?

 

[amfmsw]

Uh, fellas, you forgot to mention that it isn't actually "Stereo", but as the "mathematics the inmotion" explanation put it, it's Multiplex. You'll see the MPX selection on your old Fisher, Scott, Harmon-Kardon and early FM "stereo capable" tuners. Marantz, the GOOD stuff made in Long Island NY, called it Stereo, as my Marantz 10B does, and that was 1964.

 

[radioskeptic]

The multiplex output jacks on old mono tuners were detector outputs (usually buffered, low-impedance cathode-follower outputs) of the signal without de-emphasis. They were intended to feed an outboard stereo demodulator and sum-and-difference matrix unit as soon as the FCC settled on a system, but could also feed an outboard SCA unit.

Whether a station broadcasts an SCA, a stereo-difference signal or both, the correct term is multiplex. The stereo-difference (L-R) signal itself isn’t stereo, but it’s what you need to recover the left and right channels. It’s only one form of multiplex operation.

BTW, the term “multiplex” is also used in multiplex (carrier) telephony and multiplex telegraphy, where multiple conversations or telegraph messages are sent over single pair of wires. Any time you simultaneously send two or more streams of intelligence – and I didn’t say “streams of data,” since each stream may be either analog or digital – over a single radio transmitter or a single wire circuit, that’s multiplex operation.

 

[700WLW]

The multiplex output jacks on old mono tuners were detector outputs (usually buffered, low-impedance cathode-follower outputs) of the signal without de-emphasis. They were intended to feed an outboard stereo demodulator and sum-and-difference matrix unit as soon as the FCC settled on a system, but could also feed an outboard SCA unit.

Whether a station broadcasts an SCA, a stereo-difference signal or both, the correct term is multiplex. The stereo-difference (L-R) signal itself isn’t stereo, but it’s what you need to recover the left and right channels. It’s only one form of multiplex operation.

BTW, the term “multiplex” is also used in multiplex (carrier) telephony and multiplex telegraphy, where multiple conversations or telegraph messages are sent over single pair of wires. Any time you simultaneously send two or more streams of intelligence – and I didn’t say “streams of data,” since each stream may be either analog or digital – over a single radio transmitter or a single wire circuit, that’s multiplex operation.

Hippo,

To understand my vast knowledge of broadcast communications theory, I have no clue, what radioskeptic is talking about in the first two paragraphs. I have done work in software network protocol analysis, but that is about it.

 

[hipporadio]

The multiplex output jacks on old mono tuners were detector outputs (usually buffered, low-impedance cathode-follower outputs) of the signal without de-emphasis. They were intended to feed an outboard stereo demodulator and sum-and-difference matrix unit as soon as the FCC settled on a system, but could also feed an outboard SCA unit.

Whether a station broadcasts an SCA, a stereo-difference signal or both, the correct term is multiplex. The stereo-difference (L-R) signal itself isn’t stereo, but it’s what you need to recover the left and right channels. It’s only one form of multiplex operation.

Hippo, To understand my vast knowledge of broadcast communications theory, I have no clue, what radioskeptic is talking about in the first two paragraphs. I have done work in software network protocol analysis, but that is about it.

radioskeptic is making a fairly straight-forward explanation. Your difficulty relating to his post may be more about your age then your ability to understand. What he is describing initially bordered on ancient FM history, and hasn't been common on consumer FM tuners since the 70s. If you're a younger guy, you wouldn't have had the occasion to even see the output he discussing on any FM receiver you own.

Back in the late 50s as FM stereo drew closer to implementation, tuner manufacturers selling top-of-the-line mono gear wanted future-compatibility with the FM stereo system once it was finally approved by the FCC. A customer wouldn’t want to spend hundreds of dollars on a hi-fi rig—only to find it “behind the curve” a year later. Basically what radioskeptic is describing is an upgrade path that allowed the owners of that gear to enjoy FM stereo on equipment which was limited at the time of its design to mono operation. This would require the purchase of an additional adaptor.

In the 1970s, the FCC evaluated several proposals for FM quadraphonic broadcasting. Quad recording fizzled and with it--so did any need to broadcast it on FM. Many tuners designed at that time featured similar outputs which could have allowed for an upgrade to FM quad.

Simply put--the detector output he describes is a “raw” export of the entire audio baseband after a selected station’s RF signal is demodulated. Consider it to be an unpainted non-accessorized car. Once you get it home--you get to choose the color and the trim.

SCA stands for “Secondary Carrier Authorization”. They are suppressed carrier “secret signals” modulated above the primary audio in an FM transmission. Like FM stereo, they are sub-carriers to the main channel information. Popular uses include (or have included) subscription background music, spoken word services for the visually-impaired, data and text streams--even slow-scan television. The digital FM system SUPERCASTER is fond of would employ use of an FM SCA (but generally at the expense of analog stereo—but not main channel mono).

 

[audiophile.]

It's very easy to add a composite output jack on tuners.

I just probe around the demod looking for the stereo signal on a scope. If the output level is low I add an op-amp buffer, and mount a BNC on the rear chassis.

 

[calstymes]

Tom Wells, radioskeptic, hipporadio and audiophile., thank you for being there. I learn so much from you guys.

Some day, when this is all just a bad memory, we will have you guys to thank for telling the IBOC story like it is.

 

[radioskeptic]

I always though SCA stood for "Subsidiary Communications Authorization."

Anyway, it might help to think of SCA as being FM radio's equivalent of stereo analog TV SAP, or Secomnd Audio Program. Although the frequency of an SAP is a little lower than that of an SCA (and the L-R difference sidebands are lower, too -- the suppressed carrier for that DSB is 31,469 cycles per second, or double the video line frequency), the technology works exactly the same way.

But there's an important regulatory difference. SCA's are not intened for reception by the general public. And while a handful of outfits catering to hobbyists have offered SCA demodulators for private use, they come with a warning that they are not licensed for use in public places. (SouthWest Technical Products offered one such kit back in the mid 1970's And BTW, what ever happened to SWTP?)

 

[amfmsw]

Also, most stations elect NOT to use their SCA because it hurts them in the loudness war.

The SCA uses about 10% of the modulation envelope, and with that envelope being "100%", it only leaves about 80% available for regular programming.

Another 9% is being consumed by the Stereo Pilot tone. The pilot tone (38Kc) is used to tell your tuner that this signal has stereo (mpx) programming, and allows the IF to seperate the L and R channels.

Even though you can't hear these things, you only get "100%". Stations running sloppy "hot" modulation or processing will make that stereo light blink on loud passeges, it's called clipping the pilot, and is illegal, but rarely enforced.

 

[hipporadio]

I always though SCA stood for "Subsidiary Communications Authorization."

Those darn SCAs... Here they come back to haunt me again! THANKS radioskeptic--your memory’s better then mine. Having had to deal with those critters at 6kw, I’m sure you’ll understand why I’d prefer to forget them.

We employed only one at 67kHz to return transmitter parameters to our Burke ARC-16 remote control. We set injection to the very lowest allowable level (3-4%) and recovered it off-air 3-miles away at the studio. The best SCA receiver we could find at the time was from Radio Systems. It was a “portable radio” made agile by cracking open the case and maneuvering several rows of DIP switches.

The Burke became an ATS when we locked the doors at midnight. It “phoned home” when an out-of-tolerance condition occurred. I remembered the misery our poor engineer endured when the band opened and our 100kw neighbors came rolling in on the adjacent channels at 4AM. He would dutifully arise to reset the Burke via his cell phone--but not without calling me to say "There IT goes again"!

Also, most stations elect NOT to use their SCA because it hurts them in the loudness war.

And that would be our stations also... I declined the temptation to even deal with RDS for that very reason.

Stations running sloppy "hot" modulation or processing will make that stereo light blink on loud passeges, it's called clipping the pilot, and is illegal, but rarely enforced.

The biggest culprit for “clipping the 19kHz pilot” was that trashy Modulation Sciences composite clipper those loudness nuts would insert into the air chain after the stereo subcarriers were formed. There was NOTHING “science” about that pathetic box--a virtual z-diode (or was it 2?) placed smack across the composite audio. Aural level ducks and so goes the pilot... And don’t forget the odd-order garbage thrown into the “back-40” of the baseband that confused some Delco and Chrysler radios into believing that “stereo blend” was necessary within site of the tower!

We DID find ONE composite proc that worked well. It was hand-made to order by Jim Somich in Cleveland. It fast-limited/filtered the baseband and provided for downstream insertion of a “clean” pilot.

We’re currently discussing audio proc analog merits vs digital demerits in the Raleigh/Greensboro forum under the thread: “Hitz/Magic, You’re Distorted”

http://www.radio-info.com/smf/index.php/topic,54445.0.html