idea for homebrew recording studio over ethernet

[Cellular Mitosis]

Hey guys,

I have been pondering an idea recently, and thought I would bounce it off some of you.

My idea is to develop a cheap solution for a multitrack digital home recording studio.

It is broken down into these peices:

- Analog to Digital Converters
- digital audio to ethernet converters
- PC with an ethernet card

Basically, you have a bunch of mics set up in a recording studio. Each mic is connected to an analog to digital converter. this ADC outputs a stream of digital samples, in SPDIF or similar.

Each ADC connects to a "black box" which takes in digital audio samples, strips off the SPDIF headers and such, wraps them up in a IP packet, and sends them onto the ethernet.

The PC is receiving all these packets from all the black boxes, and is writing the audio tracks to disk.

This idea is cheap, because you can use commodity PC hardware and networkign hardware.

This idea allows you to have conplete control over the audio quality of your recordings, as you decide what ADC to purchase (this is the only peice of the equation which affects audio quality).

This idea is scalable, in that you can set up a 4 track recoding studio, only to later decide you need 16 tracks. You simply add more equipment, and dont lose any of you original investment.

Also, there is no reason you can't have more than one PC receiving packets, as long as you have some method of syncronizing the blackboxes. In fact, there should be no theoretical limit to the number of concurrent tracks you can record.

This idea has been sitting in the back of my mind for probably a year now, but only recently have I decided it might be feasible to implement. Take a look at the picoweb server prototype:

http://www.picoweb.net/download/ccol9908.pdf

There, they demonstrate the feasability of interfacing a microcontroller to a standard ISA ethernet card, including a full TCP/IP stack.

anyone's thoughts on this are welcome.

-jason
jason@pepas.com

[tschrama]

Nice idea... if you can get it to work.. could be very nice .. could you do a ball-park calculation off the band width needed in each node of your network.. Wouldn't you need a DSP manage all those megabits..?

thijs

[Cellular Mitosis]

well,

assuming 44k/16bit audio tracks,

44100 x 16 = 705600 bits/sec, just for the raw samples.

705600
1411200
2822400
5644800

8 tracks would require more than 5.6 Mbit of bandwidth.

this assumes zero collisions, and does not take into account the overhead of ethernet packet headers.

so, with several NICs in one machine, you could provide more tracks than you will probably need.

however, 100Mbit might be a better way, to account for header overhead and network collisions, etc.

[Cellular Mitosis]

Standard networking follows the OSI 7 layer model.

each layer is an abstraction, so that each layer doesnt have to know anything about the layers below it.

each layer wraps the data in more headers.

for an entertaining tutorial, see:
http://www.pe.net/~rlewis/Resources/james.html

7 ...
6 ...
5 ...
4 transport (TCP)
3 network (IP)
2 data link (802.3 ethernet frames)
1 physical

for efficiency, lets say for now we only use up to the IP layer.

from this:
http://www.networkmagazine.com/article/NMG20000727S0013

we see that an ethernet header is 26 bytes:

7 byte preamble
1 byte start of frame
6 byte MAC address (destination)
6 byte MAC address (source)
2 byte frame type
up to 1500 bytes (data)
4 byte CRC footer.

however, apparently the first 8 bytes is considered to be at the physical layer, and is not considered in the typical 1518 byte maximum ethernet packet?

then we have the IP header,
http://www.cis.ohio-state.edu/cgi-bin/rfc/rfc0791.html

which is at least 20 bytes.

thus, even with no samples, our packet is 46 bytes if I am correct.

if we were going for a little better than 50% efficiency, we would need 24 samples at 16 bits each, thus 48 bytes of data.

for 75% efficiency, we need 72 16 bit samples in each packet (190 byte packets)

100Mbit is looking like the way to go.

[jduncan]

uh, ok sorry........had to interject here.

First, you should ***REALLY*** try to aim for a bit rate higher than 16 bit /44.1kHz for this......in all seriousness.

Secondly, debating about whether or not to use 100MBit or not is pointless as you cant buy anything slower anymore (or if you can its obsolete and the 100MBit hardware is no more expensive, if not actually cheaper).

So do the calculations again, assuming a 100MBit network and say 24 bit/ 192kHz optimally but at least 24/96

[Cellular Mitosis]

calm down there, my plans are to make it scalable to 96/24, but I was just doing feasability calculations.

the 10mb or 100mb thing is a debate over "can I get away with using ISA NICs" or "do I have to use PCI NICs".

I think I effectively demonstrated that ISA NICs are not the way to go, as they would strain to do 8 tracks of 44/16, making the whole solution not worthwhile.

The implication here is that with PCI, not only could you use more than 8 tracks of 44/16, but the possibility of higher sample rates and resolutions is opened up.

but besides this, debating over 100 vs 10 is not "pointless", as I am sure it woudl be much easier to learn how to interface with an ISA card vs a PCI card, not to mention ISA NICs are cheaper.

hopefully future posts in this thread will be a little more constructive and polite.

[Cellular Mitosis]

to interface with the ADC, the blackbox will need a receiver chip capable of understanding S/PDIF, AES/EBU, or both.

some notes on these standards:

http://www.hr/josip/DSP/FAQ/26.html
http://www.epanorama.net/documents/audio/spdif.html

this document mentions some receiver chips:
http://tjev.tel.etf.hr/josip/DSP/FAQ/42.html

searching on google for each of those reveals the cs8412 seems to be the more popular of those.
http://www.cirrus.com/pubs/8411.pdf?DocumentID=201

however, the cs8414 and cs8413 are capapble of taking 96k/24bit input.
http://www.cirrus.com/pubs/8413-4.pdf?DocumentID=272

the cs8414 will operate with being controlled by a microcontroller. This may make for a simpler design.

[jduncan]

cellular, ok im sorry i really didnt mean to come across as rude.

I'll go back and clarify some things as well as respond.

check these two links:
ISA Price check

PCI Price Check

As you can see, PCI nics start @ $3, ISA @ $7. Big deal, either way they are both insanely cheap.

Also keep in mind, that more and more newer motherboards dont actually have ISA slots anymore so if you want this to work on all new computers effectively thats out. Interfacing the card is fine, you can make your thing run over tcp/ip, its really not that difficult to do.

The cards drivers add an abstraction removing any difference between pci and isa.

ISA is an old, outdated standard that should not be used for any new products, in a way similar to the thread about people using 2N3904's etc in new designs, its really not something you should do.

I'm sorry I offended you with that, it wasn't my intention, I'm just trying to set you straight.

[jduncan]

dammit, the url's didn't work properly......just copy and paste the link part out, but those prices i posted below are correct anyway.

[jduncan]

sorry, i just noticed that i think you were referring to the network on the adc end. the interfacing here is an issue, i'll give you that and i apologize for some of what i said before. however, if you're going to use multiple 10 MBit cards, that's pretty ridiculous.

additionally, now that we're talking about the adc end, use an all-in-one ethernet ASIC.

its much cooler.