Thread: Modern Day FMs
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Old 2007-09-04, 10:33 PM
GIGFY26 GIGFY26 is offline
 
Join Date: Jan 2007
Re: Modern Day FMs

ok, this may get a little complicated.

I'm no expert but have worked in tv for a long time.

FM radio and tv audio, which also uses frequency modulation for it's audio, are two different but related things.
Frequency modulation is just a way of transmitting a signal. Think of it as encoding in modern terms. In AM, the amplitude of the radio wave changes to carry the information when compared to a baseline. In FM it's the frequency or the radio wave that is changed compared to a baseline. This is not to be confused with the frequency / channel of the radio or tv signal over the air which is a different thing and is what determines the baseline.

Lossy really just means that all of the original audio is not being reproduced. Mp3 and others, take into account human hearing and try to get rid of the frequencies [again another usage of the same word] that are "beyond the range of the average human hearing"

Answers to your questions to the best of my ability:
Different recording methods have different FA characteristics.
FA cut off just below 15khz is often indicative of mp3 for live recordings. I am not sure of broadcast specifications that cut there, but the info can probably be found at wikipedia or hydrogenaudio. For instance web broadcasts will have different specs than satellite radio which is different from digital radio etc.
The sharp line in the SA indicates to me an electronic clipping of the signal information.
If an original source has no information above 15khz [like a classic mp3] but is broadcast over the air on a signal that has a broader frequency range, there may still be information [ie noise even if you can't hear it] above the 15khz threshold.
Think of it as the broadcast coming down a pipeline. If the water flowing only fills the bottom half of the pipe there is still air and other stuff coming down the pipe above the water line.

I don't know that FM and TV cut at 16khz. I guess I could look it up, but I can pretty much guarantee that it varies around the world. I find it a little hard to believe that FM radio in the US and TV audio use the same thresholds but I could be wrong. Also if it's using compression and not clippers, I wouldn't think it would be such a hard line on the SA.
DATs also produce cutoffs of the frequencies but it varies depending on the mode [ie LP vs SP etc]. Mini Discs have yet another set of characteristics. Generally the DAT and MD recordings have drop offs at higher frequencies. Also variable bit rate mp3s cause even more gray area.

I've been told, and I believe it, that truly recognizing and identifying all the signatures of different recordings is as much an art form as it is science. That's why having as much lineage info as possible is crucial. For instance knowing it was originally a DAT in LP or a MD give you a great jumping off point. FWIW audio ripped from DVDs is a whole different set of specs. Some is lossy but some isn't.

Reading this back I'm not sure I really cleared anything up but it's a very complicated subject and while I am experienced, audio is not my specialty.

Hope this helps. I'm really try to learn to read these myself.

Disclaimer:
This is a limited, general and sometimes not totally technically accurate description of a complicated subject. I'm am not and will not be in a flame war but any constructive input is of course welcome.
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