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ARSCLIST  August 2009

ARSCLIST August 2009

Subject:

Re: Digital broadcast (Long Response)

From:

Robert Cham <[log in to unmask]>

Reply-To:

Association for Recorded Sound Discussion List <[log in to unmask]>

Date:

Wed, 5 Aug 2009 15:37:45 -0500

Content-Type:

text/plain

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text/plain (248 lines)

Hi Charles,

Very good post, with a few minor inaccuracies.  Please pardon my 
interlineation.

>Hi Bob;
>
>Some comments on the mono issue of your last  paragraph that you 
>know but others may not.   It raised both technical and public 
>educational issues.   I appreciate and prefer that mono programs or 
>signals be broadcast and received in mono for the reasons you gave. 
>It is a cleaner and quieter signal with greater dynamic and 
>reception range.  
>There is the inherent technical limitation of the FM stereo system 
>the FCC mandated for the US in the 1960s.  The FM stereo performance 
>is inferior to mono as far as SN,  frequency response, phase, etc is 
>concerned.  (I do not know about Europe FM stereo, whether its the 
>US FM stereo system or something different  and better.  I think its 
>the same FM stereo system, the difference is the pre and de 
>emphasis, modulation limits, spacing between channels, and the FM 
>Band assigned frequencies.)

The only differences between American and European analog FM  broadcast are:

1)	Pre/deemphasis curves  Just like LPs, FM broadcast emphasizes 
the highs during broadcast and de-emphasizes them during reception in 
order to reduce noise which is greatest in the high frequencies, in 
the process.  This also has the effect of making it necessary to 
carefully control highs before broadcast in order to not overload the 
transmitters and receivers.

The European standard for the curve is 50 microseconds, and the 
American is 75 microseconds.  An effort was made to push the European 
standard through the FCC back in the '80s, without much success.

2)	Europe uses even frequencies for broadcast, 91.4 MHz VS 91.5 
MHz in the U. S.  No problem for older analog tuners, or the newest 
digitally tuned tuners, but whole generations of digitally tuned 
tuners cannot do it.

BTW the Sony XDR-F1HD is currently the best sounding analog tuner. 
It also has some remarkable circuitry for improving RF performance. 
It is also only $100, and an additional $30 rebate right now makes it 
very affordable.  Unfortunately it cannot be made to tune the analog 
channel if a HD channel is also present.  However, if you're handy 
with a soldering iron there is a modification available at 
http://www.ham-radio.com/k6sti/xdr-f1hd.htm.  This article also 
includes instructions for forcing mono mode, extending audio 
headroom, and flattening the audio frequency response.


>A big problem is car radio FM reception.  RF signal levels vary all 
>over the place and multi-path distortion reception issues are far 
>worse in stereo than mono.  Once I had an excellent Becker mono FM 
>car radio.  Its reception and mono sound was excellent, and its 
>audio hiss noise was  much less than newer FM stereo car radios. 
>Acoustic sound issues in cars are bad enough.  The variable and 
>often degraded FM stereo car radio reception adds more listening 
>complications.

It is possible to add space diversity reception to some car radios. 
This makes use of a separate antenna that may not be in a multi-path 
null at the same time as your primary antenna.  If you would like me 
to explain multi-path, just ask, on or off list as you prefer.  For 
that matter someone should let me know if these OT posts are welcome 
on the list.

If you wish good reception from all directions with a single antenna, 
it should be located in the center of the car's roof.  This is 
because the car is the ground reference for the receiver and as such 
it is part of the antenna system.  Mounting an antenna on a fender 
makes it highly directional, which is only desirable if one is using 
multiple antennas

The primary problem with stereo analog FM is that it requires 10 
times as much RF field strength at the receiver as mono FM.  A 
properly received signal is "fully quieted" the result of which 
should be no discernable noise except that which is added by the 
receiver.

>
>An FM receiver's audio S/N ratio varies not only with its RF signal 
>level but also to an extent with the receiver circuit design.  A 
>weakly received FM signal may have an audio S/N of 50 db in mono and 
>only 30 DB in stereo.  As the RF signal level goes up,  the S/N 
>improves, and the mono/stereo noise differences go down.   With a 
>really strong RF signal, the receiver S/N in mono can easily be 65 
>db with stereo at 55 db which is not too much worse.   In really 
>good FM tuners/equipment, the S/N in mono can be 90 db, with stereo 
>at 85 db, if there is a moderate or higher RF signal level which the 
>antenna delivers to the RF front end circuits.


There is a specification generally available for all radios that 
states the necessary RF signal strength necessary for mono or stereo 
reception with full quieting.  Look for those two  figures when 
buying a receiver.

  Once that signal strength is achieved, the limits of S/N are the 
noise generated in the receiver and the S/N of the broadcast chain. 
90 dB S/N might be achievable in a laboratory, but I've seldom seen a 
broadcast signal chain achieve 70 dB, and that was only with the 
transmitter and studios in the same building.  The Studio to 
Transmitter Link (STL) always adds noise and distortion, whether it's 
analog or digital telephone lines, radio links, or theses days even 
IP devices.

>
>WIth regard to typical off air car radio FM reception, (an important 
>market for the station,) the cards are stacked against quality 
>issues.  These car radios have limited chassis and control space 
>with cost limited circuit design.  Many are in stereo FM reception 
>mode all the time, and thus noisy.

Some can be forced into mono.  Look for them

>
>In other cases the radio can auto switch between mono and stereo.
>When that happens one can get not only stereo and mono acoustic 
>sound changes but background audio hiss noise changes too.  It is 
>rare for there to be a manual mono/stereo switch on a car radio for 
>the user to select.  Few have a threshold level adjustment control 
>for stereo reception triggering, or a blend function to make stereo 
>noise matters better when the stereo is hissy.

Most can and do.  I believe that switching by blending is universal.

>Even if the station goes to mono transmission, many car radios just 
>keep their stereo circuits going which is simple and cheap.

I've not seen a modern receiver that does not switch to mono when the 
stereo pilot is extinguished.  That function is switched by the 
absence of the stereo pilot

>Thus the improved noise benefit of mono transmission is severely 
>degraded by the receiver itself staying in a forced stereo mode even 
>with a mono signal and no stereo pilot.
>
>Then, as you say, if the FM station broadcasts a mono program in FM 
>mono for better fringe reception, when it switches back to  FM stereo
>for its stereo programs, then complaints may occur about the noisy 
>FM stereo!  The station is blamed for the  FM stereo hiss noise and 
>technical problems when its mostly a matter of the FM stereo system 
>design itself, weak signals, poor antennas, poor reception, poor 
>receiver design, and public ignorance of any of the technical issues 
>of mono and/or FM stereo transmission and reception.

When I worked in Vermont, I insisted on the following being read 
before switching to mono.  "In the interest of best reception and 
lowest noise, the following mono program will be broadcast in mono." 
There were about 10 complaints in 5 years, when that procedure was 
followed.  I believe that they later discontinued the notice, but the 
listeners had been educated.


>When FM stereo broadcast signals  are summed to mono at the receiver 
>for a better SN, then often stereo audio phase and reception phase 
>issues  occur to make the 'summed into mono' sound bad. 
>Muli-channel surround has more of these issues and problems  unless 
>it is done right in every respect.

Actually, that's not how it works.  Stereo FM consists of a L+R 
signal, a L-R signal, and a 19 KHz stereo pilot signal, that lights 
the stereo light and switches in the blend circuitry when needed.  So 
when the signal is fully switched to mono in your receiver, you are 
listening to the much more robust L+R signal generated at the 
station.  The L+R signal is simply a summation of the L and R outputs 
of the stations audio console.

If the stations audio processing is correctly set up, then any phase 
anomolies that you hear are due to the original recording.  While 
mono compatibility is the watchword of all radio broadcasters, it is 
much less so in the recording industry.  It used to be the case that 
all recording studios kept a small mono speaker to check mono 
compatibility.  It can also be checked on an oscilloscope, but you 
don't see that much anymore either.  Instead most studios keep a 
stereo pair of cheap speakers to check boom box compatibility.

Coincident micing for stereo is almost automatically mono compatible. 
Many mics in the relatively small confines of a studio, however, can 
have severe phase effects that are not as easily noticed in stereo as 
in mono.  Poor studio technique is actually at the root of most phase 
problems in mono reception.

>Radio stations failed to educate the public about these technical 
>matters.  Stations should havsaid "This mono program is broadcast in 
>FM mono.  We are turning off our FM stereo sub carrier and your 
>stereo light will go out during this mono program.
>When FM stereo operations resume, our FM stereo sub carrier will be 
>turned back on and your FM stereo light will come on.  This FM mono 
>broadcast will help fringe listeners get better reception with 
>reduced background noise and it provides lower hiss levels for all 
>our listeners."   Something like this should have been announced 
>when transmitting in FM mono mode.   Then perhaps the complaints 
>would have stopped or been reduced.  Many people wrongly thought the 
>FM stereo light had to be on all the time.  When the light was off, 
>listeners believed something was wrong with either the station's 
>broadcast signal or their FM equipment.   Even today, there are 
>programs that are mono produced - talk shows, music, etc.  These 
>ought to be broadcast in FM mono for better quality of the 
>transmitted signal and the FM mono reception.   For mono playback of 
>records, either use a mono cartridge, or sum the stereo cartridge 
>output to mono.


There was a time that the FCC required that the stereo pilot be 
turned off for broadcast of mono material.  Sadly that went away with 
the presence of engineers in control rooms, many years ago.

>Have mono set ups for tape decks,  CD players, etc. to raise their 
>sonic performance and audio quality level to what FM mono can be. 
>It is not  costly or difficult to do this.  My comments are for over 
>the air analog FM broadcasting.  I omit digital and satellite issues 
>that may or may not be similar to the above.
>
>Any ideas on how to get the broadcasters and public to do justice to 
>these occasional mono situations?

Sadly it's not practical with just DJs in the control rooms.  They 
are hired for their patter, not for their technical ability.

>One can still listen to FM mono on as many speakers as one wants, 
>just please leave real mono programs and mono FM signals alone. 
>Enjoy mono's unique qualities as best one can, just as in earlier 
>days.   Of course, its unlikely anyone will do anything.   When 
>there is real quality, there are often mistaken complaints that then 
>become destructive and counterproductive.  The quality and 
>performance  standards of good or bad are eliminated.

And they largely have been except at a very few, mostly public radio stations.

>This is the easy way to silence the complainers.  Regrettably, as 
>you indicate, the clear trend is to reduce quality issues to a very 
>low level.  But some do care, so keep up the good fight.
>
>Yours truly,
>Charles A. Richardson

Once again, thank you for your concerns.  I'm sorry not be more 
optimistic about quality standards in Radio.  That is the primary 
reason why I will be happy to retire soon.  When it comes to Europe, 
Italy at least is no better than the U.S.  I don't think that I will 
be doing much radio listening other than short wave, once I retire. 
I shall miss some programs such as Prarie Home Companion, and may 
resort to the low quality audio of web streams to listen to them.

Bob Cham

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