24 to 18 bit conversion ??? - diyAudio
Go Back   Home > Forums > Source & Line > Digital Source

Digital Source Digital Players and Recorders: CD , SACD , Tape, Memory Card, etc.

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 8th March 2006, 07:43 PM   #1
Lyra is offline Lyra  Norway
diyAudio Member
 
Lyra's Avatar
 
Join Date: Nov 2003
Location: Norway
Question 24 to 18 bit conversion ???

Is it possible to take a 24-bit (or 20...) left justified or I2S timed signal (LR alternating) and convert it to 18-bit Left Justified MSB-first (LR alternating) ?

I have been looking around here (...and the web....) without any information around this "problem"

Lyra
  Reply With Quote
Old 8th March 2006, 08:16 PM   #2
diyAudio Member
 
Join Date: Oct 2001
Location: .
Default Re: 24 to 18 bit conversion ???

Quote:
Originally posted by Lyra
Is it possible to take a 24-bit (or 20...) left justified or I2S timed signal (LR alternating) and convert it to 18-bit Left Justified MSB-first (LR alternating) ?

I have been looking around here (...and the web....) without any information around this "problem"

Lyra
It is a simple task that depends to certain extent on the source but why would you choose to truncate 6 bits?
  Reply With Quote
Old 8th March 2006, 08:38 PM   #3
Lyra is offline Lyra  Norway
diyAudio Member
 
Lyra's Avatar
 
Join Date: Nov 2003
Location: Norway
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
  Reply With Quote
Old 8th March 2006, 11:41 PM   #4
diyAudio Member
 
Join Date: Oct 2001
Location: .

With I2S and left justified data, wordlength does not matter as excess bits are simply ignored.
  Reply With Quote
Old 9th March 2006, 04:52 AM   #5
diyAudio Member
 
Francis_Vaughan's Avatar
 
Join Date: Feb 2005
Location: Adelaide
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.
  Reply With Quote
Old 9th March 2006, 07:48 AM   #6
diyAudio Member
 
Join Date: May 2005
Location: Wisconsin!
Send a message via AIM to Rescue Toaster
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)
  Reply With Quote
Old 9th March 2006, 09:15 AM   #7
diyAudio Member
 
Francis_Vaughan's Avatar
 
Join Date: Feb 2005
Location: Adelaide
Oops... Should read closer.

OK, dither is a problem. And indeed bit clock rate - that could be fixed - but the dither is not easy.
  Reply With Quote
Old 9th March 2006, 09:24 AM   #8
Lyra is offline Lyra  Norway
diyAudio Member
 
Lyra's Avatar
 
Join Date: Nov 2003
Location: Norway
The SM5847 (or similar) do some of the work I think, but the format out is on separate pins....and it should be on the same pin altenating with the L/R signal....

Am I way out here, or could this be a part of the sollution ??
  Reply With Quote
Old 9th March 2006, 09:43 AM   #9
Lyra is offline Lyra  Norway
diyAudio Member
 
Lyra's Avatar
 
Join Date: Nov 2003
Location: Norway
Quote:
Originally posted by Rescue Toaster
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)
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
Attached Files
File Type: zip dataoutputtiming2.zip (33.5 KB, 17 views)
  Reply With Quote
Old 9th March 2006, 10:02 AM   #10
diyAudio Member
 
Join Date: Oct 2001
Location: .
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.
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
SVC to DVC conversion madstamm Subwoofers 8 13th January 2008 04:45 PM
dB/W/M conversion - M vs 0.5 M ?? MadMutt Multi-Way 3 26th December 2007 12:51 PM
9v to 12v conversion purprobe Parts 2 17th September 2005 09:37 PM
rca conversion oogared Tubes / Valves 3 3rd September 2005 05:48 PM
PCM to DSD conversion liz Digital Source 6 2nd October 2003 08:53 AM


New To Site? Need Help?

All times are GMT. The time now is 09:55 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2