I want to truncate the signal because I want to replace a defective and obsolete! AD-converter.
The original AD-converter deliver 18-bit, but there arent so many (if any) 18-bits converters available, and the rest of the circuit expect 18-bit...and wouldn't/ couldn't handle more bits.
24-bits AD-converters on the other hand.....is widely available.
So...how do I manage to trunkate the signalflow from one newer and relatively good AD-converter ?
Lyra
The original AD-converter deliver 18-bit, but there arent so many (if any) 18-bits converters available, and the rest of the circuit expect 18-bit...and wouldn't/ couldn't handle more bits.
24-bits AD-converters on the other hand.....is widely available.
So...how do I manage to trunkate the signalflow from one newer and relatively good AD-converter ?
Lyra
Truncation would not in itself be difficult. A small PLA, or a handful of logic chips. On the other hand the audio will not take kindly to simple truncation. You must dither the signal again to decorrelate the new least signignificat bit. Otherwise it will not sound good at all. Doing this well is not trivial.
Edit:
Hang, on - you want to do the opposite. this should be fine - a 24 bit DAC will usually pad out and work OK.
Edit:
Hang, on - you want to do the opposite. this should be fine - a 24 bit DAC will usually pad out and work OK.
Francis, it's an ADC not DAC.
I think the problem is a 24 bit ADC might be operating at a different bitclock than the 18 bit was. IE, it possible but it might not be easy. If the old 18 bit ADC padded out to 32 bits (thus 64 bitclocks per wordclock) it should be easy. If it did something funny and run at like 36 bitclocks per wordclock it would be pretty hard.
Can you post part numbers for the old ADC chip and the next chip in the line? (whatever the ADC is sending it's data to)
I think the problem is a 24 bit ADC might be operating at a different bitclock than the 18 bit was. IE, it possible but it might not be easy. If the old 18 bit ADC padded out to 32 bits (thus 64 bitclocks per wordclock) it should be easy. If it did something funny and run at like 36 bitclocks per wordclock it would be pretty hard.
Can you post part numbers for the old ADC chip and the next chip in the line? (whatever the ADC is sending it's data to)
Should read closer.
Rescue Toaster said:
The old chip is a CS5389-KP, and it sends the data into the DSP in an Lexicon PCM90 (via a 74HC08), and I am quite sure that I can do nothing with the way the DSP expects the signal...it's way to complicated to alter !!?
I am posting a zip-file containing a piece of the Data Output Timing chart regarding the old AD-converter the way I think it is used in this perticular equipment
Just about every audio ADC runs SCLK at 64Fs in at least one format and so long as the CS5389 you are replacing was set to I2S or Left justified data, the wordlength will take care of itself.
From audio quality POV though, simply chopping off 6 bits is less than ideal. The SM5847 will not, however, solve your problem. It is an oversampling filter and I doubt the circuitry downstream from the CS5389 was designed to cope with 8x oversampled data.
If you cannot find an 18bit dac try a 16 or a 20 bit one. The PCM1801 and PCM1800 from TI are current production 16 and 20 bit dacs respectively. TI also have an 18 bit dac but it is considerably more expensive than any other ADC and requires an external decimation filter.
From audio quality POV though, simply chopping off 6 bits is less than ideal. The SM5847 will not, however, solve your problem. It is an oversampling filter and I doubt the circuitry downstream from the CS5389 was designed to cope with 8x oversampled data.
If you cannot find an 18bit dac try a 16 or a 20 bit one. The PCM1801 and PCM1800 from TI are current production 16 and 20 bit dacs respectively. TI also have an 18 bit dac but it is considerably more expensive than any other ADC and requires an external decimation filter.
rfbrw said:
The ADC You are referring is PCM1750 I presume ?
I have looked at this but the appropriate decimation filter (DF1750) is no longer available from TI. Is there any other usable filters available ? I can't find any !?
When it come to cost...if this is what it takes to get it working...this is the way I have to go...
Whould it be better to use a 16-bit ADC instead of a 20 or higher ?
(....will not loose any data or something like that ???)
Lyra
I can get (I think....
) CS5330A in small quantities, an for what I can see this little ADC could be just perfect for this, but I am a little concerned about the dynamic range...it's just 94dB, versus the original CS5389's 107dB (both A-weighted)
This is PA equipment and I am a little concerned that 94dB just might be a little small number !?
) CS5330A in small quantities, an for what I can see this little ADC could be just perfect for this, but I am a little concerned about the dynamic range...it's just 94dB, versus the original CS5389's 107dB (both A-weighted)This is PA equipment and I am a little concerned that 94dB just might be a little small number !?
I'd use a TI TAS3103. It is a simple little DSP which can input and output virutally any format at virtually any bit depth. And it can provide you with dithered 18 bit signal if you wish, rather than truncated. You do not need to write a single line of code to use this DSP; you just configure it.
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