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Old 1st June 2004, 05:23 PM   #1
Schaef is offline Schaef  United States
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Default DAC basics?

I recently got some recievers, DAC chips and filters with the intention of builing my own external DAC to play around with. My first problem is understanding the data sheets. I don't understand how you know which sampling frequency to choose and how everything operates.

The datasheet talks about how to select a clock frequency based on fs, how do I know? If I use a reciever capable of recieving up to 24/96, does it automatically pick the input speed and everything else falls into place if set up correctly?

Is there a primer somewhere that talks about what all of the things the datasheets talk about mean? They all seem to assume a certain basic knowledge that I don't appear to have.

Thanks for any help!
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Old 1st June 2004, 08:11 PM   #2
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That basic digital audio knowledge can be learnt by doing an electronics course or by doing it the hard way; trying and doing wrong followed by repairing and testing . The latter method costs more time and money and is pretty inefficient but more focussed on the area you want to explore.

Both methods include a lot of reading and require a basic but thorough understanding of how electronics work. Most manufacturers of digital audio chips have other documents besides their datasheets like application notes etc. that can be a source of valuable information.This site can be of help too. If you wander through the Digital section you'll find enough links on the matter. You'll get to know the Search function as a bonus

My advise is to save the documents/datasheets ( some manufacturers only offer their datasheets as long as the part is in production !!! ) you want in folders on your harddrive for future reference. I generally convert them to pdf for readability. After one harddisk crash you'll appreciate the backup you made to CDR as I found out the hard way ...
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Old 2nd June 2004, 04:41 AM   #3
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You can start with basic settings like 256 fs and 16 bit. Have jumpers or switches to make changes afterwards. Then you can try as many things as you like.
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Old 2nd June 2004, 12:20 PM   #4
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Schaef,

I've been thinking in a similar manner recently, as I decided to attempt my own MP3 player.

Of coarse, to the everydayer, that's sounds impossible. But, if you look around on the net, there are already a lot of sites about building your own MP3 player. For instance -

YAMPP - Yet Another MP3 Player

I recently downloaded an article from Circuit Celler by Jan Syzmanski titled 'Build Your Own MP3 Player'.

Although it involves perhaps a more than average level of digital knowledge for the programming side and so forth, it does include some useful descriptions about memory usage that even I can follow.

It's definitly worth the $1.50 to download it.

A method that I have found highly beneficial is the simple rereading one.

Read and read until your mind start to ache, then do something else for a few days. When you come back to look at it the second or third time, it makes a lot more sense, even if you had absolutely no idea what any of it meant the first time. It's almost as though your mind is subconciously building the dictionary of words, terms and relationships that you'll need to know, without evening knowing their true meanings.
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Old 2nd June 2004, 01:31 PM   #5
Schaef is offline Schaef  United States
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Default Thanks for replies!

Thank you all for your replies! I guess I should clarify a couple of things, first I've been reading the digital forum for quite some time, so I thought I had an idea of what goes on. Digital sampling makes sense to me and I understand from a computer perspective. (I'm a programmer in the real world) My problem is that the basics behind these are a little unclear and maybe I'm worrying about something I don't have to. I've been looking at schematics of the evaluation boards and other DIY DACs I could find and trying to understand them.

So, let's take a quick step back, and start with a couple of basic questions then.

1) given a reciever chip (say DIR1701 from TI) that's good up to 96kHz. Do I have to tell the chip what data rate to expect coming in or does it auto detect the rate?

2) Once the data has gotten from the reciever inside, what do the different fs settings do in the DAC? Is there a way to select one MCLK and accept 44.1, 48 and 96? (I know one MCLK can be used for 48 and 96, but what about 44.1?)

So, I think my biggest concern is with choosing a MCLK rate that allows the greatest flexibility and understanding how to choose it.

Thanks for any additional help!
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Old 2nd June 2004, 02:10 PM   #6
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It's also useful to have a look at the data on the various DAC, digital receiver, and ASRC chips from people like Analog Devices and Crystal (Cirrus). The evaluation boards usually are complete DAC modules and the data sheets have full schematics and usually a lot of other helpful information.

I've just bought the AD1896 ASRC evaluation board from Analog Devices. CS8414 rx, AD1896 ASRC and AD1852 DACs (As well as co-ax and opto SPDIF receivers with a co-ax SPDIF driver on the board). Basically a high-end digital rx/ASRC/DAC/digital tx board at less than 100 UK pounds.

Data on the AD web site under part number EVAL-AD1896EB
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Old 2nd June 2004, 03:03 PM   #7
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Quote:
Originally posted by Ouroboros
It's also useful to have a look at the data on the various DAC, digital receiver, and ASRC chips from people like Analog Devices and Crystal (Cirrus). The evaluation boards usually are complete DAC modules and the data sheets have full schematics and usually a lot of other helpful information.

I've just bought the AD1896 ASRC evaluation board from Analog Devices. CS8414 rx, AD1896 ASRC and AD1852 DACs (As well as co-ax and opto SPDIF receivers with a co-ax SPDIF driver on the board). Basically a high-end digital rx/ASRC/DAC/digital tx board at less than 100 UK pounds.

Data on the AD web site under part number EVAL-AD1896EB
Why an AD1852 DAC, and not a 1955?
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Old 2nd June 2004, 03:09 PM   #8
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The 1852 is the DAC that AD have put on the evaluation board. Looking at the specification for that DAC, it looks as if it will be quite good enough for my ears!

I only got the evaluation board yesterday and haven't had a chance to try it out yet. (I need to build or buy a really nice case for it and build a suitable +/- 12V PSU to power the board.)
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Old 2nd June 2004, 04:19 PM   #9
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I set my DIR1701 up with a 12 Mhz crystal for 256 fs and it automatically syncs to different rates like 32k, 44.1k, and 48k as winamp plays them.
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Old 2nd June 2004, 09:30 PM   #10
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I'm probably going totally off track now, but my excuse is that I'm as new to this and only trying to help...

Doesn't the DAC take it's processing rate from the clock channel? I mean, for instance, in the MP3 player schematics I'm looking at, the clock is a global part.

It feeds to the microprocessor and the DAC.

When the DAC's DREG, data request, channel signals so, the microprocessor starts reading the data out of the memory and sending it out, syncronised with the positive or negative edge of the clock cycle.

The DAC then uses the same clock channel to sense when it should be receiving new packets for unpacking and decoding.

In the DAC arrangement I was considering, the clock is an external 12MHz crystal. As I say, the clock's output then went to both the microprocessor and the DAC.

The PDF of the DAC made sure to say that I should avoid using third harmonic clocks, as these would cause timing failures in the system.

If you're not going to use an independant memory and processing system, and just clip the DAC onto your desktop, surely you will only need to connect the clock of your DAC to the clock of your desktop to syncronise the data feed correctly? This is where I check out, since I have no idea which pins the clock is sent out on, a search on google should bring up some piccies though.

I don't think that this particular DAC could automatically sync it's self.

As I say, I'm not professing to even be close to some of you guys when it comes to digital, so I'm ready to be wrong. I just thought this might help.
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