Fundamental Questions: ADCs, DACs, I2S - diyAudio
Go Back   Home > Forums > Design & Build > Construction Tips

Construction Tips Construction techniques and tips

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 4th August 2011, 04:05 PM   #1
diyAudio Member
 
Join Date: Jan 2010
Default Fundamental Questions: ADCs, DACs, I2S

I have a question which google (despite my trying) has not been able to answer. I hope someone here with more knowledge than I would be gracious enough as to correct my understanding on the following point:

I2S data is converted and output by a DAC relative to that specific device's own 0dBFS level and as nothing to do with the voltage the signal was originally sampled at.

example: If an ADC (0dBFS = 2.5Vpp) samples a 1Vpp signal and outputs this signal via I2S to a DAC (0dBFS = 5Vpp), the DAC would output an analog signal at 2Vpp.

Is this correct?
  Reply With Quote
Old 4th August 2011, 04:12 PM   #2
kevinkr is offline kevinkr  United States
diyAudio Moderator
 
kevinkr's Avatar
 
Join Date: Sep 2004
Location: Boston, Massachusetts
Blog Entries: 6
Yes..
__________________
"Extraordinary claims require extraordinary evidence." - Carl Sagan
  Reply With Quote
Old 4th August 2011, 04:17 PM   #3
diyAudio Member
 
Join Date: Jan 2010
Thanks for the speedy reply, greatly appreciated. One simple follow up: does anyone know which DACs have the highest 0dBFS level available?
  Reply With Quote
Old 4th August 2011, 04:28 PM   #4
diyAudio Member
 
analog_sa's Avatar
 
Join Date: Aug 2002
Location: Sofia
Some dacs have current rather than voltage outputs which makes answering your question more complicated.
  Reply With Quote
Old 4th August 2011, 04:39 PM   #5
diyAudio Member
 
Join Date: Jan 2010
Good point, sorry about that.

The current setup is designed around a voltage output DAC, however I can not get get the final level required for the project without resorting to large amounts of analog gain (40 dB) after the DAC. This raises the noise floor beyond what is acceptable at lower listening levels. I was hoping to overcome this by utilizing a DAC with a higher output in the first place.

If a current output DAC can offer a significantly higher output and lower noise, I would be happy to consider it.
  Reply With Quote
Old 4th August 2011, 04:44 PM   #6
diyAudio Member
 
Join Date: Jan 2010
I should note that this is a unique project with unusually high output level specifications, and that a minimum full volume output of about 10Vpp is required
  Reply With Quote
Old 4th August 2011, 09:04 PM   #7
diyAudio Member
 
Join Date: Nov 2009
Location: Los Angeles
Quote:
Originally Posted by rsbonini View Post
I should note that this is a unique project with unusually high output level specifications, and that a minimum full volume output of about 10Vpp is required
So worst case you add an opamp at the DAC output to get whatever level you want.

G
  Reply With Quote
Old 4th August 2011, 10:34 PM   #8
diyAudio Member
 
Zero Cool's Avatar
 
Join Date: Sep 2004
Location: MN
I have s somewhat related question...is there a standard or a reference voltage level for 0dbfs??? IE should 0dbfs = say 2vrms? or is there a standard for what 0dbfs is voltage wise?


Zc
__________________
It's not about what you can BUY...It's about what you can BUILD!
  Reply With Quote
Old 4th August 2011, 11:18 PM   #9
jcx is offline jcx  United States
diyAudio Member
 
Join Date: Feb 2003
Location: ..
monolithic audio DAC, ADC use mixed signal CMOS IC processes - it is handy to use "standard" digital supply V of 5 V, anaolg reference can be close to that in some chips trying for maximum dynamic range, further trying for maximum dynamic range usually involves differential in/out


V output CS4398 DAC is specd as giving 120 dB dynamic range with 6.65 Vpp differential output

you should put a buffer/filter op amp after any audio DAC, differential to SE conversion can include gain



a pretty common desktop consumer digital audio analog output reference level is 2.0 Vrms = 0 dB fs

a technical oddity is that some bit patterns can produce a "over 0 dBfs" on the filtered/reconstructed analog output - a guard band of 3-6 dB would be necessary to avoid this potential problem

Last edited by jcx; 4th August 2011 at 11:23 PM.
  Reply With Quote
Old 4th August 2011, 11:33 PM   #10
diyAudio Member
 
abraxalito's Avatar
 
Join Date: Sep 2007
Location: Hangzhou - Marco Polo's 'most beautiful city'. 700yrs is a long time though...
Blog Entries: 103
Send a message via MSN to abraxalito Send a message via Yahoo to abraxalito Send a message via Skype™ to abraxalito
Quote:
Originally Posted by rsbonini View Post
The current setup is designed around a voltage output DAC, however I can not get get the final level required for the project without resorting to large amounts of analog gain (40 dB) after the DAC. This raises the noise floor beyond what is acceptable at lower listening levels. I was hoping to overcome this by utilizing a DAC with a higher output in the first place.
From what you say about too much noise at lower listening levels, you have the additional gain after the volume control. Moving it prior to the volume control would mean at lower listening levels, the noise also would be lower.
__________________
No matter if we meanwhile surrender every value for which we stand, we must strive to cajole the majority into imagining itself on our side - Everett Dean Martin
  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
Couple of fundamental questions.... dvdvideo Multi-Way 33 15th June 2005 08:27 PM
DSD ADCs and DACs Brett Digital Source 8 14th October 2002 08:59 AM


New To Site? Need Help?

All times are GMT. The time now is 09:23 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