Brian
Brown [ ] diyAudio Member
Offline Registered:
Aug 2002 Location:
Wisconsin
|
High Resolution
Multi-Channel Digital Interface |
Post
#1 |
This is an update on my experiments with a
real-time direct-digital audio interface.
Highlights
of my present implementation:
- Excellent sound
quality (IMHO). - Multichannel support (six channels). -
Good electrical noise immunity. - ESD protection. -
Power supply sequencing between interconnected units can occur
in any order. - Tolerant of live plug-in and
disconnect. - Master (low jitter) clock can be located next
to DAC or digital amplifier. - Flexibility with multiple
source formats (DSD, I2S, RJ24, various sampling
frequencies). - Destination format is standardized at 24bit
I2S (sampling frequency up to 200KHz, dependent on master
clock). - Low cost, standardized interconnect cables and
connectors (CAT5). - Suitable for short-distance (six inch
box-to-box) as well as longer distances (I’ve tried up to
twenty-five feet, but it should be able to go even
further).
Background:
A couple of years
ago, I was inspired by the availability of new digital
amplifier chipsets, audio processors, as well as the coming of
multichannel DVD-A and SACD. I had long wanted to implement an
all-digital signal path, including digital speaker crossovers
and room correction. Digital amplification offered a more
practical implementation of the large number of separate
amplifiers that I would require. I’d already formed a rather
strong opinion that for a given cost, a digital amplifier
could offer better sound quality than a traditional DAC /
analog amplifier combination.
I spent most of the first
year studying digital amplifiers and working on some
preliminary designs. It became very apparent that the lack of
a standard commercial multichannel digital interface was going
to be a roadblock to both my digital processing and digital
amplifier experiments.
I bought a Panasonic SA-XR10
receiver as an experimental platform after discovering that it
utilized TI’s Equibit digital amplifier chip set. One of the
things that really startled me was how much better CDs and
2-channel DVD-As sounded through S/PDIF compared to using the
analog link: Disk player >>> DAC >>> ADC
>>> Digital Amp. (I knew that bypassing this analog
patch job was going to be an improvement, but I didn’t expect
it to be such a big one.)
This revelation caused me to
drop everything else and focus my attention on a new digital
interface.
Considerations:
The present
S/PDIF standard only supports Stereo PCM (up to 96KHz) without
compression. I wasn’t interested in compressed multi-channel
formats such as AC3, DTS, MP3, etc. (If I happen to want to
play an AC3, DTS, or MP3 disk, I’ll use the player to decode
it before sending it over the interface.) Also, S/PDIF uses an
imbedded clock that has some data-dependant clock recovery
issues.
The only direct-digital SACD and DVD-A
interfaces that I’m aware of are proprietary to certain
manufacturers and will only work with their
equipment.
The upcoming IEEE-1394 digital audio
interface requires a tremendous amount of overhead. It
probably will offer excellent sound quality, but it will be
very DIY-unfriendly.
USB looks promising for DIY. TI’s
TUSB3200A supports eight channels of USB to PCM interface.
Their reference design should help reduce the work required to
get this up and running. Still, it requires a lot of overhead.
I may reconsider it in the future, but I decided that I wanted
to stay with a synchronous interface for now.
LVDS over
twisted-pair appeared to be the best type of signaling to use
for an interface. There are many support chips available for
it, as well as it becoming a standard on newer FPGA’s. It
isn’t dependent on a standardized supply voltage, it has good
noise immunity for this type of application, the timing
accuracy is very good, and the bandwidth exceeds the
requirements of digital audio.
Initially I’d thought to
use discreet parallel LVDS links for each of the I2S lines.
The biggest problem with this is that for multi-channel, it
would require expensive cable and connectors. There also could
be a problem with timing skew between the links, especially
with longer distances.
A Serializer-Deserializer
(SERDES) approach offers quite a few advantages over parallel:
Multiple signals can be sent over a single twisted pair. This
allows a much cheaper CAT5 cable and modular connector system
to be used. Since the data is reclocked, buffered, and
synchronized at the receiving end, SERDES is virtually immune
to skewing, and offers additional jitter
rejection.
SERDES can be implemented either with a
clock embedded into the data (a single LVDS pair required), or
a clock separate from the data (two LVDS pairs required).
(Most SERDES chipsets with imbedded clocks have managed to
avoid the data-dependant clock recovery problems that are
associated with S/PDIF.)
The SERDES transmitter
requires a control clock that is synchronous to and a multiple
of the data being transmitted. In this case that clock would
be the MCLK of the disk player. Most disk players use either
256fs or 384fs for their MCLK. The possible frequency range
for MCLK then becomes 11.2896MHz (44.1KHz CD w/ 256fs) to
73.728MHz (192KHz MLP DVD-A w/ 384fs). This turned out to be
the primary consideration in choosing a particular SERDES
chipset. The SERDES’ PLLs need to be able to operate over this
frequency range.
The only SERDES chipset that I was
able to identify with this PLL frequency range was TI’s MuxIt
devices. This is a four chip solution for one data link: a PLL
(SN65LVDS150) and a Multiplexer (SN65LVDS151) for the
transmitter side, and a PLL (SN65LVDS150) and Demultiplexer
(SN65LVDS152) for the receiver side. MuxIt uses separate LVDS
clock and data pairs. It can be configured to pass between
four and ten parallel data lines (not including
MCLK).
A CAT5 cable has four twisted pairs. A MuxIt
link requires only two of these pairs. I use one of the
extra twisted pairs as a ground link between the transmitter
and receiver units. The jury’s still out on whether it’s
better to have this ground connection or not. It can prevent
the LVDS signals from exceeding the common mode range of the
devices, but it also could possibly introduce a ground loop.
So far, I haven’t experienced any problems with it connected
or disconnected.
There are a couple of possible uses
for the fourth twisted pair: remote power, or a master clock
signal to the transmitter from the receiver. This master clock
could be used to synchronize the transmitter (such as a disk
player) to the DAC or digital amp. There are two constraints
with this: the transmitter would have to be able to synch off
of an external clock, and the receiver would need to know what
frequency the transmitter required (this might change for
different disks). Because these constraints make it harder to
implement a ‘universal’ interface, I choose not to send back a
master clock at this time.
I tried running a MuxIt link
from a DVD-A player to the SA-XR10 digital amp, using the
recovered clock sent from the DVD-A to clock the Equibit
section. Not only was I able to hear multichannel surround
without the analog link for the first time (wow!), I found
there to be a substantial sonic improvement on regular stereo
CD’s compared to the S/PDIF interface (the thing that inspired
me to focus on this exercise in the first place). I realized
that the S/PDIF link on these units was hardly optimized (lots
of board-to-board interconnects, etc.), but I still was
surprised. To help convince myself that this wasn’t purely
psychological, I A/B’d the interfaces (using a button on the
remote) for several people. Everyone preferred the MuxIt
interface, saying that they could hear better detail, or that
things sounded clearer. By contrast, the difference going from
the analog link to S/PDIF was more along the lines of improved
depth and imaging. (I want to avoid debates on comparison
testing or subjective descriptions. This is what I tried, this
is the best I can do to describe it in
print.)
(continued in part 2...)
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