Fred E - N8UC wrote:

"The audio quality on the FM IBOC channels is about as good as a 256K
MP3. An analogue FM transmission is capable of MUCH better sound
quality (although only a few stations such as WFMT in Chicago or WQXR
in New York actually broadcast uncompressed FM analogue sound)."

Here we have to be careful about the term 'compression', especially in
tems of audio quality and digital audio through-put. 'Compression' in
the analog audio sense defines the process of 'squashing' an audio
signal - reducing its dynamic range - in an effort to make it of a more
consistent level. This is felt to be important by most radio
broadcasters for competitive reasons; the 'louder' stations are felt to
have more 'dial presence' and therefore are more able to capture
listeners scanning the dial. It is true that some high-powered
classical music stations employ very little compression, and yet analog
FM is always hampered by the 75 us audio pre-emphasis and the 10%
injection of the stereo sub-channel (which requires a hefty received
signal to get anywhere near a 60db S/N ratio - a pretty poor showing).

'Compression' as it relates to digital radio refers to the ammount of
(audio) data that is 'thrown away' due to the need to fit the signal
into the available bandwidth. The ubiquitous "MP3" at 128 kbps has
"thrown away" about 90% (ninety percent) of the material that was found
on the origianl (uncompressed) Compact Disc. The 'computer' programs
that perform this task are running a 'compression algorithim', and much
work is and has been done in this field. It is the basis of *ALL* of
the current crop of consumer / portable / personal music devices (Ipod
/ cell-phone audio / Internet Radio etc).

To illustrate: Fred E quantifies his IBOC FM experience as beig
similar to a 256 kbps MP3. In reality, FM IBOC stations are limited to
a total data through-put of 98 kbps, 2 kbps of which are reserved for
overhead and data. Hence, Fred E was listening to a 96 kbps signal.
Quite remarkable, really especially in a direct A-B comparisson between
most (uncompressed) source material and the recovered IBOC signal.

AM IBOC is limited to a bit rate of only 32 kbps (!) - about 95% of the
original data has been tossed overboard, and yet... Fred E and I will
have to disagree on the aural result, which has a lot to do with the
care the radio station has taken in their audio and RF infrastructure
(IBOC *mandates* a very flat antenna system, something most AM stations
have not historically had to consider. Hence, ones aural experiences,
especially in these young days of AM IBOC may vary). I too am a vetran
of C-Quam AM stereo, and IMHO IBOC has a much lower noise floor and
much better stereo separation.

Lastly, the "specifications" page of Fred E's Yamaha receiver (nice
unit!) :

http://www.yamaha.com/yec/products/r...4600_specs.htm

completely ignores the AM section. Hummm... could this be the trouble
with your AM IBOC reception...?

Mike Worrall
Los Angeles