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Radio Bitrates?

post #1 of 3
9/23/08 at 6:49pm
Thread Starter 
What is the equivalent bitrate to a signal of FM radio? is it constant or does each station pick theyr sound quality? it seems to me that some radios sound better than others. before you say that its because the transmitters are further away, dont. The two radios in question share the same transmiting pole thing.
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post #2 of 3
9/23/08 at 9:47pm
Quote:
Originally Posted by NoHands View Post
What is the equivalent bitrate to a signal of FM radio? is it constant or does each station pick theyr sound quality? it seems to me that some radios sound better than others. before you say that its because the transmitters are further away, dont. The two radios in question share the same transmiting pole thing.
You can estimate the approximate Audio information [1] content of FM by using the bandwidth and dynamic range. FM pretty much tops out at about 15K and the dynamic range is about 70db. Each bit represents approx 6db so we need just under 12 bits and a sampling rate of 30K.

Thus = 12 x 30,000 x 2 (2 channels) = 720Kbps (CD is 1411kbps) that is uncompressed.

1. FM transmission is somewhat more complex than that with carriers and what have you but we are not interested in that level of accuracy.
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post #3 of 3
9/24/08 at 5:00am
This question is also complicated by the fact that virtually all radio stations nowadays use lossy compression extensively in their production process. So from a quality standpoint, it really could be far worse than you might think.

Jukebox software using Musicam/MP2 (!) has been standard for quite some time now - I remember seeing a Windows box with a sound card control all the programming on a station in 1992 or so, with a 9GB RAID array for music storage. NPR, if I'm not mistaken, still uses MP2 for its satellite feeds. The BBC has taken great pains to ensure a lossless signal path to the station, although it's typically compressed to hell on DAB anyway.

Nowadays I understand that syndicated content, and even individual music singles, are transferred over FTP, typically using MP3 or something of that nature. And it's entirely unclear what bitrates and encoders they're dealing with.

Ultimately, this hobbles the sound quality of HD Radio. The bitrates I've seen for it are great, and soundly beat XM/Sirius down - but virtually everything played on radio is already lossy compressed. So digital FM radio generally requires a transcode! Not good.
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