I have gained much valuable insight from reading the posts from last month on
Linux Audio Dev and OpenGraphics. I have tried to incorporate what wisdom I
could. Someone wiser than I could greatly improve on this, of course. Hence
this document.
Low to mid-range markets are saturated. Buildable, but not likely saleable.
Top end market is pretty tight. R&D budget for that level of gear is probably
more than the video card project. Semi-pro is more doable.
Best bet is an aggressive design to sell on performance and features. Most
advanced features can be implemented in software, so the hardware should be
fairly straight-forward, but it still needs good throughput.
Target market
Semi-pro audio (high end of target)
High end gamers (mid range of target)
Home theater (low end of target)
Hobbyist
-Hobbyist add-ons could greatly increase saleability to other markets
US$300? price range for baseline system - this needs to be priced out
19" rackmount boxes may cost more. Mainly due to the sheer volume of good
stuff that can be packed into them. see below.
Features
192kHz @ 24bit inputs and outputs (48 bit fixed point internal)
on board resampling/mixing
sample rate
bit depth
# channels
channel swap
on board realtime compression/decompression/transcoding
all analog I/O is in (shielded) breakout boxes
more than one kind of box supported (including custom boxes)
more than one box at a time supported
breakout boxes may be up to 100m from host system
MIDI
5.1 optical
ADAT
sound samples/effects may be preloaded to card and played/mixed on demand
full 3D audio processing
custom codecs and effects generators may be loaded on demand
on board speech synthesis - just another codec :)
on board speech recognition - same :)
IR remote control?
watchdog timer?
Several people have voiced desire for 192kHz capability and have indicated
media at this quality is available from some sources.
Native sample capabilities
192kHz at 24 bit max - more than this would be overkill, and would hurt other
capabilities without benefit
96kHz, 48kHz, 44100Hz, 22050Hz, 11025Hz for good measure
-there is no point resampling to a higher rate unless you are going to mix
with a higher rate source. digital resampling does not increase sound quality.
-resampling from 44100 (CD audio) and lower to 48k or higher creates artifacts
(not an even multiple)
-more processing power/bandwidth is available for effects/additional channels
at lower sample speeds. halving the sample rate doubles the processing power
available per sample.
48 bit fixed point internal DSP for best accuracy
all samples converted to 48 bit fixed point for internal processing
-there should be no data loss from this conversion
-if the hardware is efficient:
-there should be negligible performance penalty for conversion
-there should be no performance penalty for using the higher bit depth
-simplified design if all DSP functions and codecs use the same format
-downsampling from 48 to 24 bit for output is trivial if implemented in
hardware
World clock
world master capable
may also use external world clock
-card can repeat clock to host system and breakout boxes
if no dedicated world clock wiring is desired or possible, soft world clock
may be implemented
-ie card and each peripheral generate their own clock and a periodic sync
signal is provided over data channel. see below.
-unit may not be able to run at full speed due to timing constraints/clock
skew in this case. not acceptable for live work, but OK for lower markets.
Virtual back plane for maximum flexibility (any input may be mapped to any
output with any filtering/effects)
external effects box can therefore be patched in at any location, if desired
volume controls are 10-12 bit analog inputs
sample at 20Hz?
volume control sampling does not need to be synchronized to any world clock
mutes are 1 bit input arrays. same sampling as above.
I/O
card is purely digital
all analog and most digital I/O is in breakout boxes. see below.
Analog
inputs and outputs are discrete from each other (no dual I/O jacks to toast a
mic with)
powered outputs are discrete from line outputs
1/8", 1/4", RCA, balanced XLR as appropriate
inputs
digitally controlled analog gain or preamp for each channel
analog prefilters to reduce digital aliasing
digital post filter? (cropping excess digital bits is trivial)
outputs
digitally controlled output gain or postamp
5th or 7th order Bessel filters standard
Digital
optical S/PDIF in and out (5.1 audio)
ADAT (lightpipe) in and out (8 channel, optical)
does anybody have specs on this?
how hard is it to get interface hardware?
cost?
licencing? (this is an open design, so this could be an issue)
MIDI
CD digital audio in/out (2 channel, 16 bit, 44100, twisted pair)
this would be on-card, if provided
could be used to link to other cards
Breakout Boxes
more than one type of box should be supportable
5 1/4" internal/external box?
cheapest to build
shielded for internal mounting
line in/line out/mic/headphone/IR/MIDI
5.1 line out?
volume control input
may only need 100baseT if bandwidth isn't a problem
9" (approx) external box
home theater/gamer
5.1 analog and optical in/out
5.1 analog speaker out (power amp)
(5.1 analogs can be configured as 3 stereo pairs, or 6 mono)
IR/MIDI/mic/headphone
software controlled AM/FM radio tuner?
LCD display?
digital spectrum analyzer display?
19" rack mount (someone with experience could probably give better layout)
I/O patch board
60-80 ports
1/4", balanced XLR (with switchable phantom power), headphone
1-2 volume controls per column
ADAT in/out
MIDI
world clock in/out
1000baseT
Control box
8-16 ports
1/4", XLR (with switchable phantom power), headphone
ADAT in/out
MIDI
world clock in/out
32 volume control sliders with mutes (2 banks of 16)
4 master volume sliders with mutes (same as above, just labelled
differently?)
knob switches? (to select option or backplane presets, etc)
2 16 pos switches could be encoded in a byte or 4 4 pos switches, etc
digital spectrum analyzer display? (also a programmable output)
-this would suggest an equalizer somewhere would be in order too :)
peak/RMS volume readouts? (which they are is a setting)
1000baseT
more than one box at a time should be supported
master can command boxes to forward streams to each other directly?
multi master?
ie more than one card connected to a group of breakout boxes and each other
more complex design
allows cards to talk to each other directly or share I/O resources
1000baseT connection
NOT TCP/IP or IPv6 - should NOT be WAN routable
don't need the extra overhead
somebody is going to want to use the same switch their internet connection
goes through....
all analog audio channels are 24 bit
volume control channels are 10-12 bit
mute controls are bit arrays
requires in-box microcontroller
as much work as possible should be done in hardware
minimal processing requirements due to simple protocol
ECC encode/decode/fixup is a large part of workload
no DSP capability, other than perhaps hardware filters
any digital filtering should be done by Master unit
simple remote commands
box detection broadcast (true PnP)
per channel input signal forwarding (either to output on same box, output on
other box, or to Master)
world clock mode selection
set gain controls
box identification
manufacturer
model
serial number(optional)
I/O count/descriptions
read current settings
main load is data stream
latch and marshal input signals for transmit (controlled by world clock)
unmarshal packets to output signal latches
2 stage latch stepped by world clock
lesser load is volume/mute update stream. same profile, just lower bandwidth
I/O bandwidth
192kHz at 24bit is 576000 B/s
48kHz at 24bit is 144000 B/s
max channels for 24 bit audio
192kHz 48kHz
10baseT 1 6
100baseT 17 69
Firewire 69 277 (400MHz)
1000baseT 173 694
100MHz could barely handle 17 channels @ 192kHz
if you add ECC (usually a good idea) it would be even less
Firewire can handle up to 69 channels (less for ECC), but protocols/drivers are
a mess
-how much does firewire hardware cost compared to 1000baseT? how to get?
-Firewire2 at 800MHz might be a possibility, but again, cost/availability?
Master Unit
1/2 height 1/2 length card
PCI or PCIe form factor
PCIe
should be main focus
offers higher bandwidth/lower latency than PCI
common on newer machines
by the time this gets to market, this will be the dominant standard
x1 should be fast enough
PCI
provided for legacy support
large current install base
limited lifetime
1000baseT external connector
100 or 1000baseT internal connector (depending on cost and whether the 5 1/4"
box needs 1000baseT)
world clock in/out (to connect to another card in the same computer)
ADAT I/O?
5.1 optical out?
RAM
32-64 MB (128?)
DDR
dual channel?
CPU
200MHz minimum
can we do 300MHz, or even 400?
overclockable with active cooling?
3rd party fan/water cooling mounts
dual core?
RISC like instruction set
48 bit fixed point DSP instructions or DSP farm
8-32 bit integer math (48 bit integer math?)
bitwise functions
should be efficient for compression/decompression using various codecs
32 bit address path so memory is upgradeable just by modifying the PCB
I/O controls/registers should be separate bus, so memory map is linear
minimal onboard boot loader, if required. all software is loaded from host
system
separate data and instruction L1 caches
MMU so codecs run in own memory, don't direct have access to hardware
necessary for buggy or untrusted codecs
DMA to access host computer memory
accelerated task switching
multiple register files?
OS
embedded Linux to start with (at least until design is proofed)
a true RTOS would be preferable
multi-tasking
SMP capable if CPU is dual core
zero copy IO for performance
will take a while to write :P
maybe hack Linux? that is what it is for, after all
on board watchdog timer?
good if she locks up
can be used as host computer watchdog timer as well
hardware random # generator?
can be used for white noise production
3 profiles of signal handling
Live
this is usually going from an analog input to an analog output
highest priority in processing
minimal buffering to reduce delay
needs to be < 20ms, even if signal is passed through card more than once
eg input -> card -> external effects box -> card -> output)
this puts the highest load on the card
critical for audiophiles
may be important for gamers and hobbyists
minimal importance for home theater
Real-time
this could be recording or playback
precise timing is only necessary at source or output, respectively
double buffering to improve efficiency is an asset
even 500ms delay playing an MP3 doesn't matter. prebuffering several seconds
wouldn't hurt anything either.
video sync isn't affected if the timing signal back to the computer is
generated AFTER the decompression codec output buffer
double (or more) buffering during recording is critical to prevent data
loss, especially if the audio is being compressed in real time
Transcode
lowest priority - this could be done entirely using time left over from the
other operations
buffering to improve efficiency is an must
delay, clock skew, etc don't matter
could be used with non-audio data with appropriate codecs (SETI at HOME,
anyone?)
If I have missed or messed up anything, that is just me being me. Please feel
free to correct/mock/browbeat as appropriate. :)
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