[BC] Digital Intgerference (was AM Interference)

[Robert Orban]

At 05:58 PM 12/17/2005, you wrote:
>From: VJB <wa3vjb at yahoo.com>
>Subject: [BC] Digital Intgerference (was AM Interference)
>To: broadcast at radiolists.net
>Message-ID: <20051217135213.61624.qmail at web52805.mail.yahoo.com>
>Content-Type: text/plain; charset=iso-8859-1
>
> >I know I speak for a number of us that this thread
>should more correctly be slugged "Digital
>Interference."
>
>Bob, to your point --
>-------
>I don't know what's wrong on the East
>Coast...(comparing to western stations)There is simply
>more punch.
>Bob Orban
>-------
>Aren't all of the qualities you cited available on
>analog AM if the station were to be set up properly?
>That's the bogus part of adding a digital overlay on
>Standard Broadcast channels: The proposed technology
>offer no technical or practical advantage to what's
>already available.  I'd sure like to see permission
>for a 30khz bandwidth (upper end at 15kHz) now
>consumed by these overlays. THEN you could start
>making apples to apples comparisons between
>transmitted signals. To work on the receive
>shortfalls, restoring this bandwidth would allow
>pre-emphasis curves where they match the typically
>poor quality of consumer radios. Type acceptance
>standards could address models now coming to market
>and specify improvements in fidelity. Nothing I'm
>saying is new, but the more I see the interference
>problems caused by digital, with no known advantage in
>quality in the field, I'm convinced this is being
>approached the wrong way.

My post did not refer to AM. It referred to comparing HD FM to analog FM 
reception. Rich Wood reported when he listened to HD FM stations in New 
England, he could either hear no difference between the analog and digital 
FM signals or the digital signal was badly processed by comparison to the 
analog. He had nothing positive to say about his HD FM listening experience.

In contrast, now that I have my Boston Acoustics Recepter, I verified that 
on many San Francisco HD FM stations, one could hear significantly more 
transient punch on drums and other percussion in rock and hip hop music 
when the radio switched from analog FM to HD reception (to the point where 
it sometimes sounded like a different mix), although the basic tonal 
balance and non-percussive texture remained similar. This is an entirely 
expected result if one is using an 8400 or 8500 to process both the analog 
and digital signals because they share all processing elements up to the 
final limiter chain. (In the 8500, the mix of the five compressor/limiter 
bands can be set interdependently for the analog and digital chains, but 
the default to have them be identical.) The HD channel requires far less 
peak limiting than the analog channel to achieve loudness parity during 
receiver crossfades. This translates as a very audible increase in snap and 
punch on transients, most of which survives the codec.

Getting equivalent punch from analog FM would require average modulation to 
be reduced somewhere between 6 and 10 dB. This number is more than 5 dB 
because the FM pre-emphasis increases the slope and magnitude of transient 
waveforms and make the processor's limiting/clipping system work even 
harder than it would have to work if there were no pre-emphasis.

This result was no surprise to me, because I have been comparing the HD and 
analog FM processed outputs of our processors for over three years in the 
lab. What is still a mystery is why Rich Wood heard none of this on any 
station he auditioned.

Bob Orban