Harris Superciter Exciter

[Drummerskip]

We are having some issues with low end audio (voices but not music) being slightly distorted on the air. Our source material is good and we have tried 2 different processors and get similar results. Our STL is a Mosley DSP 6000 system which is prone to issues, but we have moved over to a Comrex Bric Link II and we have the same issues. Out modulation is good (105%).

My question is could it be something with the Superciter? We have a Digaciter at the XMTR site but it's not in service. Our modulation going into the exciter is around 110 % I think, Could that be part of the problem?

Thank you in advance for any ideas you might have. I am only about 1.5 years into Engineering and learning as I go.

My next step is to contact Gates-Air but thought I would ask around.

Thank you.
Matthew in Sacramento

 

[dc2bluelight]

We are having some issues with low end audio (voices but not music) being slightly distorted on the air. Our source material is good and we have tried 2 different processors and get similar results. Our STL is a Mosley DSP 6000 system which is prone to issues, but we have moved over to a Comrex Bric Link II and we have the same issues. Out modulation is good (105%).

How are you measuring modulation?
Why do you think 105% is good?

My question is could it be something with the Superciter? We have a Digaciter at the XMTR site but it's not in service.

Anything is possible, but likelyhood is something else. Is the other exciter not in service because it's broken? Might be worth a try, but it's less likely the exciter is the actual cause.

Our modulation going into the exciter is around 110 % I think, Could that be part of the problem?

To quote FCC CFR47 Part 17.1570:"
(2) FM stations. The total modulation must not exceed 100 percent on peaks of frequent reoccurrence referenced to 75 kHz deviation. However, stations providing subsidiary communications services using subcarriers under provisions of §73.319 concurrently with the broadcasting of stereophonic or monophonic programs may increase the peak modulation deviation as follows:

(i) The total peak modulation may be increased 0.5 percent for each 1.0 percent subcarrier injection modulation.

(ii) In no event may the modulation of the carrier exceed 110 percent (82.5 kHz peak deviation)."

So the first question, how are you measuring modulation, is very important. An error there can result in severe overmodulation. The second question referenced to the rule above would be, "Why are you doing 110% into the exciter?" Is there SCAs?

Main channel modulation above 100% can cause distortion in some receivers. So what receiver are you listening to when you hear your problem.

Lastly, these kinds of issues are often easier to find with measurement. What measurement tools do you have available? Minimum would be a calibrated modulation monitor and some means of distortion measurement, which these days is any computer, a good sound card, and free software. Modulation meters on today's exciters are usually fairly close, but not always trustworthy.

Thank you in advance for any ideas you might have. I am only about 1.5 years into Engineering and learning as I go.

My next step is to contact Gates-Air but thought I would ask around.

Thank you.
Matthew in Sacramento

GatesAir will likely have the same questions, but would emphasize that their exciter wouldn't be the cause of distortion. I agree, but it might be part of the problem. Overmod will cause distortion in receivers, and is not legal anyway. I'd verify mod levels are real first with an accurate measurement, then adjust accordingly.

 

[rfburns]

The Superciter hasn't been in production for several years. They tend to get noisy once the electrolytic capacitors start drying out. Verifying modulation with a known good modulation monitor and looking at the output with a spectrum analyzer would be the first step.

 

[wavo]

It would be a good idea to first look at the audio going into the audio processor before you look any further.
Used to be stations were required to "proof" out the station once a year. Frequency response and distortion were checked. In your case, find a night where you can run some low frequency tones through an input to the board. Use a scope to look at every stage in the audio chain to look for distortion of the sine wave. If you're hearing distortion it is probably greater than 3% which will be a visible difference of the sine wave before and after. It may be the top of the sine waves is "flat." You would then need to check levels interstage throughout your audio chain. If everything is ok until, for example, you look at a distribution amp output, and the input is ok, then you have found the culprit. Check power supplies, individual amps, etc.
If your audio is clean to the audio processor inputs you might want to call the audio processor manufacturer and asked for advice on setting up the processors for minimal bass clipping. Clipping the pa-jeebus out of the bass is common...square waves seem to be in vogue these days! You may simply need help on reducing bass clipping.
You're shooting yourself in the foot with over deviation. It takes 1 db increase in RMS level increase to even be noticed by listeners and no one, outside of radio programmers, care about being "loud" anymore.

 

[dc2bluelight]

Actually, take a bit of time to do some simple analysis and documentation first. Trace out the entire signal flow, mic to exciter. You'll probably do a bit of tug-n-pull wire tracing, but it'll be worth the trouble. It will help you understand where the bodies are buried.

I'll agree, mostly, with the comments above by wavo. However, scopes are rare these days, computers are not. Like I said before, any computer with a good sound card will get you numbers with free analysis software. I've been using Room EQ Wizard (REW - google it) for just about every audio test, and with a $100 USB sound card I get figures that beat my old Tek TM500 stuff. I'll read out THD directly when running the RTA feature. Bit of a learning curve, but it sounds like learning is what's necessary in any case.

Inspecting sine waves for distortion is a bit hard to do unless there's a lot of it, or some form of clipping. But it's also likely transient in nature, so unless you run tones at progressively higher levels you may not find the problem.

What's standing out here is distortion on voices, not music. That doesn't mean music isn't being distorted too, but its harder to hear because music tends to mask all sorts of evils, where speech does not. And tones don't hide much of anything.

I'll take a guess that it's a processing issue first, mic/preamp/mic proc issue second, exciter last, but get those mod levels back to legal too!

 

[Kelly A]

We are having some issues with low end audio (voices but not music) being slightly distorted on the air. Our source material is good and we have tried 2 different processors and get similar results. Our STL is a Mosley DSP 6000 system which is prone to issues, but we have moved over to a Comrex Bric Link II and we have the same issues. Out modulation is good (105%).

My question is could it be something with the Superciter? We have a Digaciter at the XMTR site but it's not in service. Our modulation going into the exciter is around 110 % I think, Could that be part of the problem?

Thank you in advance for any ideas you might have. I am only about 1.5 years into Engineering and learning as I go.

My next step is to contact Gates-Air but thought I would ask around.

Thank you.
Matthew in Sacramento

Hi Matthew,

I'm not going to b*tch you out about modulation. That exciter can do 150% before you should hear any audible distortion.

The Harris Superciter could be considered their 'discounted' version of the Harris Digit exciter. Basically the same guts, just without the built-in AES inputs to an internal stereo generator. Like the Digit, the Superciter is prone to aging capacitors in the modulator section which will definitely affect audio performance. Before throwing a bunch of parts or processing at this issue, I would recommend that you connect an audio oscillator directly into the composite audio input of your exciter, run it up to 100% modulation and look at the demodulated waveform from a good FM tuner on something like a simple oscilloscope at various frequencies. That would prove without a doubt whether the exciter is clipping at certain frequencies.

 

[wavo]

I forgot to ask in my post before...are you using any type of mic processing? Is it a single mic that exhibits low frequency distortion or all of the mics? Does all voice material sound bad? Does recorded voice material sound OK?
It may be possible that a "boomy" mic is overloading the mic preamp on the low freqs. Or if processing is being used, a bad processor or bad processor settings. Check for roll off filters on your mic and try rolling off the low end and see if that helps.