Digital volume control question....
 

"Don Pearce" wrote in message
...
On Fri, 19 May 2006 16:01:35 +0100, "Keith G"
wrote:


(Ooh! Does this mean you've got ping-pong balls for eyes? :-)


I did have last weekend when I was headbutted in the nuts by a
two-year-old.


Nasty...

(When that happens it's faster than a lightning strike, innit? :-)


No, not referring to valves vs SS - that is just preference.


Agreed.



More the
fact that there is nothing more to be had from SS by spending more
money. The plateau starts *really* low.


Yes, well under a hundred quid from I can see of it!!


Two things, I think, have steered the modern trends - the inexplicable*
need
for unnecessary, loud, pistonic bass in (paradoxically) a small 'user/wife
friendly' enclosure! Consequently amplifiers have to be capable of
outputting three figures of totally unnecessary and expensive watts (in
the
average UK room) to get 'em to work at all and that brings a raft of other
considerations into the equation which are simply not of much consequence
when driving sensitive speakers with low-power amps. (Distortion, power
supply issues etc.)

Expensive watts? No, those watts are really, really cheap, especially
for subwoofers that use switching supplies and power amps.



Well, I reckon a 30 wpc amp will work out at about 2 to 4 quid per watt, but
the trouble is you *need* 100+ watts these days (ludicrous) and then I
reckon you are looking at 5 times that sort of money *at least* for similar
(construction/appearance/appointments) 'assembly line' amps....??



Interesting that there's nothing particularly small about 'high end'
speakers, innit?

Some friends of mine have Willson Maxx speakers. They weigh about half
a ton and sound equally good when bending the walls or barely
murmuring. They also stand about five feet high.


My point entirely..... :-)

Digital volume control question....
 

"Serge Auckland" wrote

Its revised my views completely, my advice to anyone looking for good
sound on the cheap would/will be 'build a pair of speakers and then chuck
any old kit at them' - I reckon you could get a superb tuner/CDP system
going for less than 200 quid!!


This reminds of the ludicrous situation that was common in the mid
eighties when Linn in particular suggested a Linn-Ittok-Asak combination
for use with a little Nytech or NAIM amp and Linn Kan 'speakers. The
results were truly horrible entirely due to the appalling 'speakers. I
really felt for people who had wasted their money in that way but what
could a layman do when every mag recommended such folly.


Missed that era - too busy working (and misbehaving) to worry about *hifi*!!
(Which is how I managed to avoid getting swept up in the 'CD - perfect sound
forever' mass hysteria!!)



When I was in retail at the time I tried to fight against it, putting
'speakers first, then decent amplification and a CD player, but went bust
for my pains. Ah well.....


For some strange reason most dealers and punters put the emphasis on the
amplifier and call it the *heart* of the system - this day and age, it's the
*last* thing to worry about, as has been noted in this thread (and others)
recently!!

........Unless it's got blue LEDs, of course....!! ;-)

Digital volume control question....
 

"Serge Auckland" wrote


I would have thought that as soon as the word "digital" is mentioned many
valve amp owners will run a mile.....


Nah - don't take too much notice of my responses to Arny's valve/vinyl
attacks!! ;-)

Although I can't ever see myself sitting down to a CD of an evening (wot a
peculiar notion???), I often have them on during the day - not to mention
DAB radio and MP3s when the occasion deems suitable!!

In a few minutes I'm orf down the road and will be playing a CDRW rip of an
LP (wot else?) on my newly-fitted 'Chav Blue' car CD player!! (The first one
I've ever had - looks like crap and sounds brilliant!! :-)

See it in the box he

http://www.apah69.dsl.pipex.com/show/thompsontwins.JPG

Heejus, innit? :-)



There are a number of passive volume controllers available albeit at
ludicrous prices for what is basically a good quality pot in a tin box;


Been there, done that:

http://www.apah69.dsl.pipex.com/passives/passives.htm

(Ended up ripping the pot out to replace the duff one in one of my Chinese
300B SETs and haven't got round to replacing yet!!)


and not forgetting the multi-tapped transformer controller which will have
all the benefits and cons of a digital controller, but again at vastly
increased cost and price.


Not going there, not going to do that.....

Digital volume control question....
 

"Keith G" wrote


Well, I reckon a 30 wpc amp will work out at about 2 to 4 quid per watt,
but the trouble is you *need* 100+ watts these days (ludicrous) and then I
reckon you are looking at 5 times that sort of money *at least* for
similar (construction/appearance/appointments) 'assembly line' amps....??


No, that stinks - what I meant was a 30 watt amp will cost somewhere around
a hundred nicker (or even less, it seems), a 100+ watt amp will likely cost
500 quid or more. (ie 5 times the price!!)

Digital volume control question....
 

Serge Auckland wrote

I would have thought that as soon as the word "digital" is mentioned many
valve amp owners will run a mile.....

There are a number of passive volume controllers available albeit at ludicrous
prices for what is basically a good quality pot in a tin box; and not
forgetting the multi-tapped transformer controller which will have all the
benefits and cons of a digital controller, but again at vastly increased cost
and price.


The DSP solution would appear to be preferable. However, presumably there are
rounding errors. That is, if I divide every word by a constant, each result must
be rounded to the nearest step, and this rounding error is not linear wrt the
audio signal. I assume there is a name for this kind of error? How significant
is it?

A digitally controlled attenuator chip contains not only a resistor ladder, but
also a heap of semiconductors to do the switching. I expect those who object to
them are wary of the SS junctions in, and perhaps also parallel to, the signal
path.

A motorised pot may be an expensive component, but it is easier to program the
control system. The pot remembers where it is, and only needs 2 bits to control.
OTOH, it is not convenient to use if you want a rotary control on your remote.

I have never seen a remote with a rotary volume control. Why not? If the link is
reasonably error free, then it should be possible to put a rotary encoder on the
remote as well as on the system case. I hate push-button volume controls.

As for what valve aficionados might think, look he

http://stiftsbogtrykkeriet.dk/~mcs/index.html

cheers, Ian

S.

Digital volume control question....
 

Ian Iveson wrote:


The DSP solution would appear to be preferable. However, presumably there are
rounding errors. That is, if I divide every word by a constant, each result must
be rounded to the nearest step, and this rounding error is not linear wrt the
audio signal. I assume there is a name for this kind of error? How significant
is it?

The DSP solutions I'm familiar with operate either floating point or 32
or 48 bit internal which means that when the output is finally reduced
to 16 or 24 bit the errors from the DSP calculations are minimised. I
don't know of a specific name for the errors resulting from DSP operation.

A digitally controlled attenuator chip contains not only a resistor ladder, but
also a heap of semiconductors to do the switching. I expect those who object to
them are wary of the SS junctions in, and perhaps also parallel to, the signal
path.

A motorised pot may be an expensive component, but it is easier to program the
control system. The pot remembers where it is, and only needs 2 bits to control.
OTOH, it is not convenient to use if you want a rotary control on your remote.

The main problems with all pots, motorised or otherwise is tracking over
a stereo pair. With 5.1 surround, there would have to be 6 tracking
controls so some form of electronic volume is almost essential.


I have never seen a remote with a rotary volume control. Why not? If the link is
reasonably error free, then it should be possible to put a rotary encoder on the
remote as well as on the system case. I hate push-button volume controls.


I don't think I've ever seen one either. The closest was a rotary
shuttle control for a S-VHS VTR which would allow frame by frame forward
or backwards movement. Now that I think about it a bit more, didn't QUAD
have one on their 66 and 77 series?

S.

Digital volume control question....
 

On 2006-05-19, Serge Auckland wrote:
Ian Iveson wrote:

The DSP solution would appear to be preferable. However, presumably there are
rounding errors. That is, if I divide every word by a constant, each result must
be rounded to the nearest step, and this rounding error is not linear wrt the
audio signal. I assume there is a name for this kind of error? How significant
is it?

The DSP solutions I'm familiar with operate either floating point or 32
or 48 bit internal which means that when the output is finally reduced
to 16 or 24 bit the errors from the DSP calculations are minimised. I
don't know of a specific name for the errors resulting from DSP operation.

I guess a digital volume control can just be a single fixed-point
multiply.

If you have (for example) a 16-bit coefficient and multiply by the 16-bit
audio value you get a 32-bit result.

Growth in significant bits is typical of DSP actions such as add and
multiply. If you just truncate the result back to 16 bits you do indeed
find that the quantization error you introduce is correlated with the
signal. It is significant and sounds bad at low levels.

In a good DSP (volume control) the remedy is to keep all significant
bits throughout the entire operation (or at least keep enough) and add
dither noise before you finally truncate back to the desired word size.
The random dither de-correlates the quantization error from the signal.

I assume that's what good DSP volume controls do.

--
John Phillips

Digital volume control question....
 

On Fri, 19 May 2006 19:02:31 +0100, "Keith G"
wrote:

Well, I reckon a 30 wpc amp will work out at about 2 to 4 quid per watt, but
the trouble is you *need* 100+ watts these days (ludicrous) and then I
reckon you are looking at 5 times that sort of money *at least* for similar
(construction/appearance/appointments) 'assembly line' amps....??

Untrue for subs, Keith. You can buy a 500 watt 'plate' amp with active
crossover and all necessary connections and controls, for less then
three hundred quid. Just the job for getting deep, clean bass down to
20Hz at decent SPLs from an 18" cube. Also one of the last remaining
areas where the home builder can beat the commercial stuff.
--

Stewart Pinkerton | Music is Art - Audio is Engineering

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Digital volume control question....
 

Serge Auckland wrote:
Ian Iveson wrote:


The DSP solution would appear to be preferable. However, presumably
there are rounding errors. That is, if I divide every word by a
constant, each result must be rounded to the nearest step, and this
rounding error is not linear wrt the audio signal. I assume there is a
name for this kind of error? How significant is it?


The DSP solutions I'm familiar with operate either floating point or 32
or 48 bit internal which means that when the output is finally reduced
to 16 or 24 bit the errors from the DSP calculations are minimised. I
don't know of a specific name for the errors resulting from DSP operation.



But even if the internal calculation is done with larger resolution, you
still loose one bit of output resolution for every 6db of attenuation.

--
Nick

Digital volume control question....
 

In article , John Phillips
wrote:
On 2006-05-19, Serge Auckland wrote:
Ian Iveson wrote:

The DSP solution would appear to be preferable. However, presumably
there are rounding errors. That is, if I divide every word by a
constant, each result must be rounded to the nearest step, and this
rounding error is not linear wrt the audio signal. I assume there is
a name for this kind of error? How significant is it?

The DSP solutions I'm familiar with operate either floating point or
32 or 48 bit internal which means that when the output is finally
reduced to 16 or 24 bit the errors from the DSP calculations are
minimised. I don't know of a specific name for the errors resulting
from DSP operation.

I have tended to see it called by various terms from "quantisation error"
and "truncation error" to "we''ll ignore this..." :-)

I guess a digital volume control can just be a single fixed-point
multiply.

If you have (for example) a 16-bit coefficient and multiply by the
16-bit audio value you get a 32-bit result.

Yes. In principle you can do that. Indeed, I think that many consumer chips
simply use such 'extended' int values for processing in places like the
digital filtering.

Growth in significant bits is typical of DSP actions such as add and
multiply. If you just truncate the result back to 16 bits you do indeed
find that the quantization error you introduce is correlated with the
signal. It is significant and sounds bad at low levels.

In a good DSP (volume control) the remedy is to keep all significant
bits throughout the entire operation (or at least keep enough) and add
dither noise before you finally truncate back to the desired word size.
The random dither de-correlates the quantization error from the signal.

I assume that's what good DSP volume controls do.

Dither will certainly help avoid such problems. However you can also employ
noise shaping. This essentially 'remembers' the quantisation errors and
redistibutes the information. The result is to provide output signal
pattern details which can be below the LSB, so helping to evade the
problem. Again, I think that many consumer chips do this were appropriate
as it is easy to build into the silicon.

The snag is that the above is all a matter of the details of
implimentation. Whereas a decent analog attenuator simply uses the
properties of the physical materials to do all this for you. No need for
the makers to work out a noise-shaping anf dithering process and ensure
sufficient precision, etc. :-)

Slainte,

Jim

--
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