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Sound chips that natively go to DC?

J

Joerg

Jan 1, 1970
0
Folks,

The subject pretty much says it. Are there any that can at least be
coaxed into DC input and output by disabling internal cancelers and
such? Should not be too drifty. The digital interface doesn't matter
much, USB, SPI, whatever. Preferably not I2C but even that would be ok.

16 bit would suffice. Of course, more is better. Input to output
synchronicity is important, as usual. Also synchronicity between the
stereo input channels.
 
M

Maynard A. Philbrook Jr.

Jan 1, 1970
0
Folks,

The subject pretty much says it. Are there any that can at least be
coaxed into DC input and output by disabling internal cancelers and
such? Should not be too drifty. The digital interface doesn't matter
much, USB, SPI, whatever. Preferably not I2C but even that would be ok.

16 bit would suffice. Of course, more is better. Input to output
synchronicity is important, as usual. Also synchronicity between the
stereo input channels.

The all do dc, you need to jump the cap inside..

as for output, that may not be such a good idea because many of
them do not operate dual rail supplies, they have dc isolation
cap.

Best thing to do is what I did years ago., AM in AM out. write the
software to decode the modulation on the input and modulate AM for the
output. You need to make an interface board, but that isn't really that
bad.

I did it via a 324 chip, one unit for the CW oscillator, a unit for
the modulator, one input from the OSC and the other input from your
reference and finally, an AM detector that uses a third unit with a
integrator in the feed back and that forms your detection. The last
unit can be used as a unity stage.

Jamie
 
The all do dc, you need to jump the cap inside..

Do you mean PC sound cards?
as for output, that may not be such a good idea because many of
them do not operate dual rail supplies, they have dc isolation
cap.

I doubt that any sound cards use dual rails. In any case, these will
have lousy DC characteristics (offset). It would take some work to
straighten that out.
Best thing to do is what I did years ago., AM in AM out. write the
software to decode the modulation on the input and modulate AM for the
output. You need to make an interface board, but that isn't really that
bad.

I did it via a 324 chip, one unit for the CW oscillator, a unit for
the modulator, one input from the OSC and the other input from your
reference and finally, an AM detector that uses a third unit with a
integrator in the feed back and that forms your detection. The last
unit can be used as a unity stage.

If you're going to go that far, why PWM from a digital port, of some
shape?
 
M

Maynard A. Philbrook Jr.

Jan 1, 1970
0
Do you mean PC sound cards?


I doubt that any sound cards use dual rails. In any case, these will
have lousy DC characteristics (offset). It would take some work to
straighten that out.


If you're going to go that far, why PWM from a digital port, of some
shape?

Not exactly sure what you mean there? are you saying creating a custom
digital port (USB for example) ? If so, that would me you need software
drivers. Something that gets messy..

Recently I was experimenting with inserting 2 FM carriers via a simple
board interface. Using both channels.

Looks interesting any ..

Jamie
 
J

Joerg

Jan 1, 1970
0
Maynard said:
The all do dc, you need to jump the cap inside..

I found that to be problematic in the ones I looked at, some sort of
DC-scrubber circuit that couldn't be disabled.

as for output, that may not be such a good idea because many of
them do not operate dual rail supplies, they have dc isolation
cap.

That would be no problem at all.

Best thing to do is what I did years ago., AM in AM out. write the
software to decode the modulation on the input and modulate AM for the
output. You need to make an interface board, but that isn't really that
bad.

I did it via a 324 chip, one unit for the CW oscillator, a unit for
the modulator, one input from the OSC and the other input from your
reference and finally, an AM detector that uses a third unit with a
integrator in the feed back and that forms your detection. The last
unit can be used as a unity stage.

I was contemplating the old HC4066 trick for chopping and synchronous
rectification but it does get old.
 
J

Joerg

Jan 1, 1970
0
Do you mean PC sound cards?

Almost, I mean the chips on there.

I doubt that any sound cards use dual rails.


That would be ok, I can take care of that. As long as the offset is very
stable and determined, like a bandgap or two.

... In any case, these will
have lousy DC characteristics (offset). It would take some work to
straighten that out.

Well, that jinxes it :-(

If you're going to go that far, why PWM from a digital port, of some
shape?


Sure, and I could also just use regular ADCs and DACs on another port.
But it makes a mess of things. Lots more parts, muxing (signals go via
the same connector), and most of all now the processor must have this on
a high priority interrupt because unlike sound chips there's no buffer.
Cost is not a big concern on this project but simplicity and size is.
 
M

Maynard A. Philbrook Jr.

Jan 1, 1970
0
I found that to be problematic in the ones I looked at, some sort of
DC-scrubber circuit that couldn't be disabled.

many of them have a small DC current put at the mic input that allows
you to power up a microphones

Jamie
 
J

Joerg

Jan 1, 1970
0
Maynard said:
many of them have a small DC current put at the mic input that allows
you to power up a microphones

Often only on one input and I can deal with that. But if, as Keith
hinted, the DC characteristics in general are lousy then I can't use
sound chips anyhow.
 
N

Nico Coesel

Jan 1, 1970
0
Joerg said:
Sure, and I could also just use regular ADCs and DACs on another port.
But it makes a mess of things. Lots more parts, muxing (signals go via
the same connector), and most of all now the processor must have this on
a high priority interrupt because unlike sound chips there's no buffer.
Cost is not a big concern on this project but simplicity and size is.

Regular sound chips (codecs) have no buffer either. The buffering
occurs in the digital interface which is part of the host chip. SPI
and I2C are likely not to work. You'd need to look for a typical
DSP-style codec interface like AC97, I2S, etc.

Removing the DC blocking capacitors should make a codec go to DC but
you'll need some differential amplifiers to make the codec deal with
ground centered signals. If you are lucky the codec outputs a
reference voltage which is used to reference the signals.
 
J

Joerg

Jan 1, 1970
0
Jim said:
What is the object? That is, define the gozinta/gozouta requirements.

Goes in: Signals from DC to 10kHz, amplitude up to 3Vpp but this gets
divided down. Input swings around ground so in single-supply chips
there'd have to be an external DC-shifter (no big deal).

Goes out: Signal of very same frequency and phase as goes it but slowly
amplitude modulated. Amplitude range much lower, generally a few
millivolts. DC stability must be impeccable. But siganl can be divided
down from FS in order to ease that requirement a bit. Modulation BW less
than 50 Hoitz and modulation depth no more than 50%.

Synchronicity between in- and output is critical and ideally the
processor should not have to take care of this. Sound chips have buffers
to deal with USB transmission gaps and the like.
 
J

Joerg

Jan 1, 1970
0
Nico said:
Regular sound chips (codecs) have no buffer either. ...


They usually do. For example, right now one from this family is in there:

http://www.ti.com/lit/ds/symlink/pcm2902c.pdf

Quote "When receiving the audio data, the PCM2900C/2902C stores the
first audio packet, which contains 1-ms audio data, into the internal
storage buffer. The PCM2900C/2902C starts to play the audio data when
detecting the next start of frame (SOF) packet, as illustrated in Figure
32".

Unfortunately it has the usual in there, poorly documented digital
high-pass filters.

... The buffering
occurs in the digital interface which is part of the host chip. SPI
and I2C are likely not to work. You'd need to look for a typical
DSP-style codec interface like AC97, I2S, etc.

Removing the DC blocking capacitors should make a codec go to DC but
you'll need some differential amplifiers to make the codec deal with
ground centered signals. If you are lucky the codec outputs a
reference voltage which is used to reference the signals.

It usually doesn't because they put digital HPFs in there.
 
N

Nico Coesel

Jan 1, 1970
0
Joerg said:
They usually do. For example, right now one from this family is in there:

http://www.ti.com/lit/ds/symlink/pcm2902c.pdf

Quote "When receiving the audio data, the PCM2900C/2902C stores the
first audio packet, which contains 1-ms audio data, into the internal
storage buffer. The PCM2900C/2902C starts to play the audio data when
detecting the next start of frame (SOF) packet, as illustrated in Figure
32".

SOF comes from USB. So yes a USB codec needs some buffering. But a
codec intended to be connected to a SoC or DSP doesn't have buffering.
At least I never came across one that did.
It usually doesn't because they put digital HPFs in there.

I've Never seen one with an HPF. Its not necessary because they are
intended to be used with DC blocking capacitors. A HPF would impair
the boom-boom sound.
 
B

boB

Jan 1, 1970
0
SOF comes from USB. So yes a USB codec needs some buffering. But a
codec intended to be connected to a SoC or DSP doesn't have buffering.
At least I never came across one that did.


I've Never seen one with an HPF. Its not necessary because they are
intended to be used with DC blocking capacitors. A HPF would impair
the boom-boom sound.


Most (if not all) of the Delta-Sigma A/D converters for audio that I
have seen and used have a HPF built in but allow you to bypass them.
For instance, this one from TI, similar to the earlier link posted but
this data sheet shows it in the block diagram...

http://www.ti.com/lit/ds/symlink/pcm1804.pdf

I'm pretty sure that the reason they put the HPF in there is because
their DC accuracy or drift is not very good. I have wanted to use one
of these regular old audio ADCs for DC before but I have gone to
slower delta-sigma converters that are optimized for DC.

I think that the old Crystal semiconductor and AKM delta-sigmas had
some kind of HPF that could not be defeated but can't remember for
sure now. The newer ones I think can be bypassed for DC operation.

One of the converters I use for DC does a DC offset calibration
in-between every conversion which works great. Probably not so easy
to do with a regular audio ADC but I'd love to be able to make one
work or hear how well an audio ADC works for someone at DC.

boB
K7IQ
 
M

Maynard A. Philbrook Jr.

Jan 1, 1970
0
http://www.ti.com/lit/ds/symlink/pcm1804.pdf

I'm pretty sure that the reason they put the HPF in there is because
their DC accuracy or drift is not very good. I have wanted to use one
of these regular old audio ADCs for DC before but I have gone to
slower delta-sigma converters that are optimized for DC.

I think that the old Crystal semiconductor and AKM delta-sigmas had
some kind of HPF that could not be defeated but can't remember for
sure now. The newer ones I think can be bypassed for DC operation.

One of the converters I use for DC does a DC offset calibration
in-between every conversion which works great. Probably not so easy
to do with a regular audio ADC but I'd love to be able to make one
work or hear how well an audio ADC works for someone at DC.

boB
K7IQ
Interesting chip. that PCM1804. my Dell XPS has the same options
on it for PCM/Digitial with the same sample rates and bits.

I'll have to check my dell next time I boot it to see what the sound
chip set is.

Jamie
 
B

boB

Jan 1, 1970
0
Interesting chip. that PCM1804. my Dell XPS has the same options
on it for PCM/Digitial with the same sample rates and bits.

I'll have to check my dell next time I boot it to see what the sound
chip set is.

Jamie


My new (ish) dell laptop uses an IDT codec and is a company I am not
familiar with. I think I found a block diagram of it somewhere but I
find that it is usually very difficult to find real documents for the
high volume chipsets that computer makers use these days.
This is especially true for Broadcom chips where they do not want the
everyday joe to be able to program them... Graphics chips too like
Nvidia. Not like it used to be where you could just get the document
for a chip and be able to program it. That's why I stick with smaller
embedded microcontroller applications where I know what is going on
inside.

Happy new Year from the yechy cold and rainy pacific northwest !

boB
 
J

Joerg

Jan 1, 1970
0
Therein lies the main problem. Even If I found a trick to disable the
HPF the DC specs are either not properly given at all or they are very
dismal. Like with the internal "reference" in some ATMegas where I
seriously wondered how someone could possibly botch an analog circuit so
badly.



An alternative would be some other AD/DA device that isn't called sound
chip but relieves the processor of having to baby its timing.

My new (ish) dell laptop uses an IDT codec and is a company I am not
familiar with. I think I found a block diagram of it somewhere but I
find that it is usually very difficult to find real documents for the
high volume chipsets that computer makers use these days.
This is especially true for Broadcom chips where they do not want the
everyday joe to be able to program them... Graphics chips too like
Nvidia. Not like it used to be where you could just get the document
for a chip and be able to program it. That's why I stick with smaller
embedded microcontroller applications where I know what is going on
inside.

Happy new Year from the yechy cold and rainy pacific northwest !

Hey, send us some of that rain. We are dying for it here, seriously.
 
B

boB

Jan 1, 1970
0
Therein lies the main problem. Even If I found a trick to disable the
HPF the DC specs are either not properly given at all or they are very
dismal. Like with the internal "reference" in some ATMegas where I
seriously wondered how someone could possibly botch an analog circuit so
badly.


I hear you ! I do not use Atmel products anymore unless I absolutely
have to after they almost put our last company out of business (early
ATmega32's) Didn't know they had a problem with V-references though.

So, one way to use an audio board for DC but with questionable upper
bandwidth is to use an (external) FM modulator. I think there was a
thread about this just recently here on SED ?

An alternative would be some other AD/DA device that isn't called sound
chip but relieves the processor of having to baby its timing.



Hey, send us some of that rain. We are dying for it here, seriously.

I now see why the elderly migrate south for the winter.
Yes, I'll trade you ! For a while anyway.
boB
 
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