How configurable is the HD encoder?

[FTL_Ian]

What bitrates are available for FM multicast in hybrid mode? Wikipedia says:

The FM hybrid digital/analog mode offers four options which can carry approximately 100, 112, 125, or 150 kbit/s of lossy data depending upon the Station Manager's power budget and/or desired range of signal (achieving perceived quality equal to a 1400 kbit/s CD). If in the future the FCC decides to discontinue analog radio, as they have done with analog television,[17] the HD Radio provides several pure digital modes. In these modes broadcasts can be made at 270 or 300 kbit/s maximum, with extra features like surround sound. Like AM, pure digital FM provides a "fall back" condition where it reverts to a low-quality 25 kbit/s signal in the event of interference.

FM stations have the option to subdivide their datastream into sub-channels (FM97-HD1, -HD2, -HD3) of varying audio quality. The multiple services are similar to the sub-channels found in ATSC-compliant Digital Television using multiplexed broadcasting. For example, National Public Radio plans to carry several different streams, calling its proposed addition to the FM standard "Tomorrow Radio". Meanwhile some Top 40 stations have added Hot AC and Classic Rock to their sub-channels to provide more variety to listeners.[18] Stations may eventually go all-digital, thus allowing as many as three full-power channels and four low-power channels (seven total). As defined by iBiquity these channels could be sub-divided into CD-quality (100 kbit/s), FM-quality (25-50 kbit/s), AM-quality (12 kbit/s), or Talk-quality (5 kbit/s) channels.

Is that info accurate? How could AM quality be defined as 12kbps when AM allegedly has 40-60kbits allocated to it?

 

[Mike Walker]

Perhaps you mean "kilohertz" (as in how much bandwidth AM has allocated), not "kilobits". "Kilobits" can't be used to measure analog waveforms, be they at radio, or audio frequencies. And AM stations sure don't have 40-60 kiloHERTZ, though HD sidebands do occupy much of that (amount of) bandwidth.

By the way, bitrates can't be directly compared for quality. In general, the higher the bitrate, the better the POTENTIAL quality. But there are so many variables, that this analogy doesn't really hold much watter. For instance, an "old" codec like mp3 may need to be as high as 128kbps to 160kbps before it really sounds good to most people, whereas a newer, more efficient codec (such as aac+ or the HDC codec used in HD Radio) can sound just as good at far lower bitrates...perhaps half (the bitrate) or less. This is why HDC was adopted for HD Radio. Undoubtedly, newer codecs will allow good audio at even lower bitrates in the future, as we learn more about human hearing, and psychoacoustics. The test of quality isn't the bitrate, it's the SOUND. If it sounds good, it is (good).

 

[FTL_Ian]

No, I mean kilobits as in kilobits per second. Again from Wikipedia:

The HD Radio AM hybrid mode offers two options which can carry approximately 40 or 60 kbit/s of data, but most AM-digital stations default to the more-robust 40 kbit/s mode which features redundancy (same signal broadcast twice).

 

[Mike Walker]

I apologize FTL_Ian, I misunderstood. I thought you were referring to analog audio in kilobits per second. My bad.