a DIY non oversampling DAC with PCM1704.
Reupload of the NONOS PCM1704 webpage. Some links may be broken. I'm sure you'll find that elsewhere.
No more further support can be given.
oversampling DACs are by many people (includung me) regarded as better sounding than
conventional oversampling systems. The easiest way to enjoy that kind of
sound is to build a DAC with the "outdated" TDA1543/TDA1541 DAC
ICs or modify an old Philips/Marantz
why not building a nonos DAC with higher quality converters like PCM1704,
PCM1702 or AD1862?
Well, you need to convert serial DATA to parallel DATA which requires some
glue logic. Further glue logic is needed to align 16bit DATA with the
20bit or 24bit inputs of these DAC ICs.
there are many well documented TDA based nonos projects available on the
www, almost nothing can be found about building a nonos DAC with
more recent Burr Brown / Analog Device ICs. Thatís why Iíll
try to give some advice and share some of my experience in building
such a DAC. If there wouldn't be this glue logic issue, I guess more DIYer would build nonos
DACs with Burr Brown chips.
paper does not provide a complete schematic for a DAC. Not even a
schematic for interfacing decoder A with DAC X. But it should support and
encourage you in designing such an interface for your own non oversampling
of all itís important to decide whether youíre going for a separate
DAC (fed with SPDIF) or a one box CD player. The one box solution (without
definitely to be preferred. Itís a little better sounding, cheaper and more
practical (imho). Of course you can add an input reciever to your CD
player and use it also as an external DAC.
are basically two kinds of CD players. The Sony and the Philips
based engines. Many decoders (SAA7210/CXD25XX/Ö)
used in those players put out a non oversampled (1fs) true 16bit signal as
stored on the CD. On the most common input recievers you have access
to different output formats, including Sony format, Philips I2S and
usually some more.
decoders and input recievers put out the
following three signals which need to be connected to the DAC ICs:
System tact, letís the converters know, at which time to read the
Letís the converter know which word belongs to left and right
Transmits two 16bit words at a sampling rate of 44.1kHz.
Transmission starts with the left word.
frequency is 2,8224MHz (44,1kHz x 64). 44.1kHz is multiplied by 64 because
there are two words per sample transmitted in a word frame length of 32
frequency is 44.1kHz (1fs). LRCK being low indicates the left word, LRCK
being high indicates the right word.
is encoded in binary twos complement (BTC) and transmitted the most
significant bit (MSB) being first. The MSB is aligned with the second
rising edge of BCK after LRCK has changed. DATA words are left justified
with delay of one BCK cycle .
frequency is 2,1168MHz (44,1kHz x 48). 44.1kHz is multiplied by 48
because there are two words per sample transmitted in a word frame length
of 24 BCK cycles.
frequency is 44.1kHz (1fs). LRCK being low indicates the right word,
LRCK being high indicates the left word.
is encoded in BTC and transmitted the MSB being
first. The MSB is aligned with the ninth rising edge of BCK after LRCK has
changed. DATA words are right justified.
decoders put out the MSB from the first till the ninth rise of the BCK.
This will lead to some distortion at low signal levels, when DATA is
shifted and fed to a DAC with more than 16bit input. With a 20bit DAC the
MSB of the next word will be regarded as the four smallest bits of the
present word. CD players using Philips decoders are preferred.
inputs of the Analog Device and Burr Brown ICs are very
is always right justified. The
least significant bit (LSB) is clocked in at the last rising edge of BCK
before latch enable (LE) goes down.
that we want to have left and right word in parallel, we always have to delay the
left word by 24 or 32 BCK cycles, depending on the word frame lenght of
step is to bring the MSB into the correct relation to LE:
using a Philips decoder, we have to delay serial DATA by another 7
BCK cycles (24bit or 20bit DAC IC). LRCK can directly be used as LE.
using a Sony decoder, we have to delay LRCK instead of DATA. LRCK must be
delayed by 8 or 4 BCK cycles (24bit or 20bit DAC IC). LRCK must be
inverted and can then be used as LE.
II: Instead of shifting LRCK, serial DATA can be delayed by another 20 or
16 BCK cycles (20bit or 24bit DAC IC). By doing so, LRCK can directly be
used as LE.
are of course further possibilities. Other formats from different input
receivers or combinations with 18bit converters. Just look closely at the
datasheets and figure out what has to be shifted and what must be inverted.
If youíve done that youíre almost there.
now is how to implement this. To create a shifter, all you
need are three different logic modules:
Flip Flop: 74HC74
datasheets will help you understand how the explanatory circuit below works. If youíve
got that, just expand this circuit to get the right amount of shifting and
Q7 (IC1) puts out the signal present on "SHIFT IN" delayed by
7.5 BCK cycles.
Q7 (IC4) puts out the signal present on its inputs (DS1/DS2) delayed by
8 BCK cycles.
The inverted clocked 74HC74 adds another delay of 0.5 BCK cycles.
A comlete example of how to combine an input
reciever with an Analog Device 18bit DAC IC can be found here.
You will never need more than 8 logic IC's to
combine any decoder/receiver with any DAC IC. It could also be done in simpler
ways, e.g. "stopped clock operation" like explained in Analog
Device application note 207 (AN207).
Or you could do it without serial to parallel conversion similar as shown here
(search for "2xPCM1702" and see post #13).
Unfrotunately it never worked with stopped clock
operation. So I tried the simple version without serial/parallel
conversion. I wasn't able to hear the phase shifts (see here
for explanation) between left and right
channel. But I wanted to have a flawless DAC, so I've implemented
serial/parallel conversion and I'm very happy with it.
Up till now I've done nonos with PCM1704,
PCM1702 and AD1862. All are built as one box CD players based on Marantz,
Denon and Philips. All of them sound superior to my tuned (also non
oversampling) Philips CD650
with TDA1541/S2 or my DDDAC1543.
Another nice idea is to build 24bit/96kHz
nonos converter for Music DVD's. When usig Philips I2S signals (or another
left justified format), you can feed anything from 16bit/16kHz to
24bit/192kHz to your converter through the same shifting interface.
Anyway, I'd say it's highly recommended to do nonos
with a really good DAC IC ;-)