Volume control [AVC?]

Rodney Kelp · May 5, 2004

You could make a million if you design an automatic volume level controller
for a televsion set. It would keep loud commercials low and low movies
higher. In other words a steady volume level. Is that asking too much from
today's technology? Even if you had to reroute the audio output of the TV
to it and run the speakers off of it. I could make a mint just installing
these things.

Lord Garth · May 5, 2004

Doesn't (didn't) Magnavox market a TV with such a circuit already...
Smart Sound isn't it?

Bob Masta · May 5, 2004

You could make a million if you design an automatic volume level controller
for a televsion set. It would keep loud commercials low and low movies
higher. In other words a steady volume level. Is that asking too much from
today's technology? Even if you had to reroute the audio output of the TV
to it and run the speakers off of it. I could make a mint just installing
these things.

Automatic volume controls are definitely non-trivial.
The problem is attack and release times. If the
gain is high for a soft part of the program, and there
is suddenly an explosion, gunshot, etc, you want the
AVC to catch it immediately, meaning it needs a fast
attack time. But then you have problems with normal
staccato things like close-up speech, since the AVC
would treat each new syllable as a little gunshot. So
you have a slow release time, to keep the AVC active
at a more-or-less constant amount for the duration
of the speech. But then after a gunshot, the gain
stays reduced for a moment. The overall effect can
be extremely annoying. Commercial units usually
have user-adjustable attack and release times, but
I'm not sure you can ever find a good compromise,
except maybe by only applying a limited amount
of AVC overall.

Bob Masta
dqatechATdaqartaDOTcom

D A Q A R T A
Data AcQuisition And Real-Time Analysis
www.daqarta.com

Soeren · May 5, 2004

Hi Rodney,

You could make a million if you design an automatic volume level
controller for a televsion set. It would keep loud commercials low
and low movies higher. In other words a steady volume level.

Most (if not all) commercials are compressed (dynamic range-wise), I
cannot think of a single movie which is, so an AGC would have to react
to the average sound pressure to have even the faintest influence.
(The volume level does not change, at least not unless you fiddle with
your remote

Some TV-sets have a preset for the volume level for each channel to
avoid the differences when zapping, but that is an entirely different
story.

[...] I could make a mint just installing these things.

Only if they worked, so start by reading up on decompression.

L. Fiar · May 6, 2004

Rodney Kelp said:
You could make a million if you design an automatic volume level controller
for a televsion set.

You really think so?
I already have a design, although it's main use is with SSB reception, to
increase the volume on the quiet signals and reduce it on the loud ones. It
uses a small IC, just a few passive components, and just 4 wires to connect
it to the volume control.

It would keep loud commercials low and low movies higher.
In other words a steady volume level. Is that asking too
much from today's technology? Even if you had to reroute
the audio output of the TV to it and run the speakers
off of it.

It's really not that complicated.
It does depend upon how the volume is controlled within the TV. If it is
controlled from the detector IC, then you need to take control of that
control voltage.
Otherwise, a small board can fit on the detector output, before the audio
amp stages.

I could make a mint just installing
these things.

Personally, I believe that the remote control makes such a device pointless
on a TV.

LF.

John S. Dyson · May 13, 2004

You could make a million if you design an automatic volume level controller
for a televsion set. It would keep loud commercials low and low movies
higher. In other words a steady volume level. Is that asking too much from
today's technology? Even if you had to reroute the audio output of the TV
to it and run the speakers off of it. I could make a mint just installing
these things.

Such devices have been available (both externally for commercial
level control) and internally (e.g. RCA 'sound logic.') One of my
good friends purchased one of the audio levelling devices for his
own TV recently (from one of those Fingerhut type catalogs.)

John

John S. Dyson · May 13, 2004

Automatic volume controls are definitely non-trivial.
The problem is attack and release times. If the
gain is high for a soft part of the program, and there
is suddenly an explosion, gunshot, etc, you want the
AVC to catch it immediately, meaning it needs a fast
attack time. But then you have problems with normal
staccato things like close-up speech, since the AVC
would treat each new syllable as a little gunshot. So
you have a slow release time, to keep the AVC active
at a more-or-less constant amount for the duration
of the speech. But then after a gunshot, the gain
stays reduced for a moment. The overall effect can
be extremely annoying. Commercial units usually
have user-adjustable attack and release times, but
I'm not sure you can ever find a good compromise,
except maybe by only applying a limited amount
of AVC overall.

My own design (works perfectly) adapts to the transient
shape of the input signal. So, if you have a 'gunshot'
and it determines that the predominant frequency components
aren't 'too low', then the attack/release is quick.
My design actually adapts to the shape of the transient
signal, and doesn't just use the spectral shape.

Another 'trick' is to make sure that the gain control is
exponential/logarithmic instead of linear... Linear gain
control tends to have problems (maintaining apparently
consistent attack/decay over wide range.)

One more 'trick' is to delay the audio signal enough that
the envelope of the gain control doesn't cause troubles.
What you do is to 'slow down' the gain control by a low
pass filter, but delay the audio signal itself to accomodate
the natural delay of the control signal. If done to extreme,
then the signal will have an apparent 'real time' delay, but
a little care will improve the sound quality.

Peter · May 13, 2004

Look for the old NE572 audio compander, still available. I build one as
stereo compressor for my TV, works great, nothing revolutionnary.

Peter

John S. Dyson · May 13, 2004

Look for the old NE572 audio compander, still available. I build one as
stereo compressor for my TV, works great, nothing revolutionnary.

For that specific application (for the TV set thing), then
the VERY old NE572 would be useful. I was also answering the
issue of 'pumping' which is practically for a different application.
(the AGC effects can be somewhat hidden by using some of the
techniques that I had suggested.) For the TV set, a simple
scheme that provides a (perhaps 10dB) gain control range, long
time constant for decay (attack would best be approx 1-10ms),
and pumping SHOULD be a non-issue.

Other AGC devices might be more appropriate if the 572 is hard
to find (e.g. other had to find analog devices components, or
even thatcorp components.)

Perhaps another approach might be to use a dual fet scheme (for
stereo), if the ICs are too hard to find.

John

Peter · May 15, 2004

John S. Dyson said:
For that specific application (for the TV set thing), then
the VERY old NE572 would be useful. I was also answering the
issue of 'pumping' which is practically for a different application.
(the AGC effects can be somewhat hidden by using some of the
techniques that I had suggested.) For the TV set, a simple
scheme that provides a (perhaps 10dB) gain control range, long
time constant for decay (attack would best be approx 1-10ms),
and pumping SHOULD be a non-issue.

Other AGC devices might be more appropriate if the 572 is hard
to find (e.g. other had to find analog devices components, or
even thatcorp components.)

Perhaps another approach might be to use a dual fet scheme (for
stereo), if the ICs are too hard to find.

John

A friend of mine, just to do it differently, picked a LM13700 (dual OTA).
After some experimentation, it's version seems to work fine.
Analog Devices makes small 8-pin chips for microphone compression using only
*one* resistor to adjust the compression ratio. I was very interested but
couldn't get my hand on one

Peter

John S. Dyson · May 15, 2004

A friend of mine, just to do it differently, picked a LM13700 (dual OTA).
After some experimentation, it's version seems to work fine.
Analog Devices makes small 8-pin chips for microphone compression using only
*one* resistor to adjust the compression ratio. I was very interested but
couldn't get my hand on one

Just another tidbit of info: there is a oft-used schemes using
LDRs (light dependent resistors), but I forget the current tradename
for them. Basically, they use the old light variable resistors with
LED with varying intensity to change the resistance.

The LDR thing is probably the highest easy quality. It is capable
of full recording studio type quality, and is trivial complexity.

The gilbert cell (or variants) including the various transconductance
schemes can have extremely variable quality all the way from 'near
perfection' on downwards to being distortion prone.

Yet another scheme would be using the variable dynamic resistance
of diodes, but there are balance issues (trying to avoid the gain
change causing interference with the rest of the signal.)

The variable gain thing is kind of a cool side-interest, but nowadays
the DSP is the most fun.

John

R. Steve Walz · May 15, 2004

John said:
Just another tidbit of info: there is a oft-used schemes using
LDRs (light dependent resistors), but I forget the current tradename
for them. Basically, they use the old light variable resistors with
LED with varying intensity to change the resistance.

The LDR thing is probably the highest easy quality. It is capable
of full recording studio type quality, and is trivial complexity.

The gilbert cell (or variants) including the various transconductance
schemes can have extremely variable quality all the way from 'near
perfection' on downwards to being distortion prone.

David Wood · May 15, 2004

Has anyone checked to see what relevant IC's Texas Instruments might
have? They seem to be cutting edge on DSP (digital signal processing)
devices.

John S. Dyson · May 16, 2004

Has anyone checked to see what relevant IC's Texas Instruments might
have? They seem to be cutting edge on DSP (digital signal processing)
devices.

Slightly diverted topic: if you have the 'energy', there is alot more
flexibility in doing audio AGC in DSP. Perhaps the biggest issue is
the input resolution (in the case of level compression), but the
troublesome issues of dynamic attack/decay times are alot easier
to implement through some simple nonlinear filter algorithms. In
hardware, the equivalent array of sythesized nonlinar capacitors
is messy.

Probably all in all -- it is likely easier to do in analog hardware
if there is no other reason for the DSP, but the DSP buys alot of
flexibility. Also, when designing the algorithms a 32bit floating
point (or bigger) can make it simpler than dealing with scaling for
the integer schemes.

John

Frank Raffaeli · May 17, 2004

Rodney Kelp said:
You could make a million if you design an automatic volume level controller
for a televsion set. It would keep loud commercials low and low movies
higher. In other words a steady volume level. Is that asking too much from
today's technology? Even if you had to reroute the audio output of the TV
to it and run the speakers off of it. I could make a mint just installing
these things.

I don't know if an audio leveler would have the effect you want.

In the 1970's I designed a box to "eliminate" commercials. It crudely
used the video "fade to black" as the trigger to signal a possible
commercial. It was still up to the user to decide what to do about it.

The Video interval test signal (VITS in RS-170A "analog") sends some
of the following information: Closed captioning, XDS (call letters,
other info) GCR (Ghost cancelling reference), network clock, and
others.

When local stations switch to their own commercials, the network clock
disappears or it is momentarily interrupted. There are other changes
in the VITS during commercials.

Perhaps some of the VITS information - along with the video content -
could be monitored to control the sound. Personally, I would like an
indicator that tells me when the program I was watching comes back
from commercial break.

Frank Raffaeli
http://www.aomwireless.com/