Firewire audio interfaces (hardware)

After my slightly disappointing experience with the Soundblaster Audigy 2 ZS I decided to step up a bit with my "project". In the past was or maybe still is an external firewire audio interface considered the best performance. The clock for audio stream is supposedly more stable than with USB and external is there less noise than inside of computer case. Therefore are all the external devices considered semi professional and come with at least 24bit and 96kHz sample rates in combination with balanced and unbalanced signal inputs and outputs to reduce noise.

The support for Firewire devices under linux is a bit sketchy:

1. As usual are manufactures of such devices not releasing drivers and or documentation to make (open source) drivers.
2. From 2007 and on went firewire the way of the dodo in favorite of USB; less computers got firewire buses (Apple stopped supporting it) and more equipment (audio, camera etc.) switched to USB as interface.
3. The ffado project is providing out of kernel drivers.
4. The Alsa sound system started to supports also a few firewire interfaces.

The second point makes that it is now a days not so "meaningful" anymore to spend a lot of time in making new or improved drivers for Firewire equipment. At the other hand this makes that it it is possible to buy fairly cheaply old firewire equipment.

I looked a bit on the ffado supported hardware and I found the following three interfaces relative cheaply (<<50 euro) on ebay.

• Terratec Producer Phase 24 FW
• Edirol FA-66
• M-Audio Firewire 410

The front and back of the three boxes are visible in the following picture:

Three (semi professional) Firewire audio interfaces

The following table gives an overview of the analogue interface functionality of the three boxes:

Brand	Type	Input				Output
		Mic	Line	Bits	Sample rate	Headphone	Line	Bits	Sample rate
Terratec Producer	Phase 24	0	2	24	192kHz	2	2	24	192kHz
Edirol (Roland)	FA-66	2	2	24	192kHz	[2]	4	24	192kHz
M-audio	Firewire 410	2	[2]	24	96kHz	2x2	8	24	96kHz

Some notes to the above shown table:

• The signal level of the headphone output of the Terratec Producer Phase 24 can be changed to line level which results in 4 line outputs.
• The headphone output of the Edirol FA-66 interface is not independent but always connected with the first two analogue line outputs.
• The M-audo Firewire 410 interface has not really 4 inputs it can use the 2 microphone inputs or the two line inputs at the back but not both at the same time.   And the two headphone outputs have independent volume control but have both the same signal.

The microphone inputs have the option to enable a 48V phantom power supply for microphones. The have a signal gain control and/or limiter. Both balanced (differential) and unbalanced (single ended) signals can be used as XLR connector or 1/4" jack plug.
At the digital side are there also some differences between the three boxes:

Brand	Type	Input			Output
		Midi	SPDIF	Optical	MIDI	SPDIF	Optical
Terratec Producer	Phase 24	[1]	[1]	0	[1]	[1]	0
Edirol (Roland)	FA-66	1	0	1	1	0	1
M-audio	Firewire 410	1	1	1	1	1	1

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Some notes to the above shown table:

• For the Phase 24 interface box are the signals available on the 9 pin sub-d connector. Unfortunately I did not get this breakout cable, but I could find the connections by some measuring. See this blog post for more information.
• The SPDIF and Optical input of the Firewire 410 cannot be used at the same time. They both are used as one data stream.
• The Firewire 410 box can use ADAT signals on the digital input which would result in 8 channels more. However I have not tried this yet.

The difference between the three audio interfaces is also very much visible when looked at the mixer possibilities. The next figure shows the three "mixer" views of ffado-mixer.

The ffado mixer interface for the Phase 24 (left), Edirol FA66 (middle) and Firewire 410 (right).

  The Phase 24 interface offers a simple mixer for the 2 inputs and the 3 output signals. This mixer can be mapped to one of the output channels. Furthermore is it possible to select the signal level of the headphone output (line or headphone), select the synchronisation source (internal or spdif) and a kind of 5 level signal gain on the analogue input.
The FA66 interface has an even simpler interface; all the options have buttons at the box itself and cannot configured over software. Only the mixer for the 4 analogue and spdif signal is left in software.
The Firewire 410 interface has the most complex mixing function to match the 2 analogue input, 2 digital input and 8 audio stream signals to the 8 analogue, 2 digital and 2 headphone output signals.
This makes it clear that the M-audio Firewire 410 box is the more "professional" one of the three interfaces. It is a pity that this specific box has only 2 analogue inputs, other, more expensive, versions have more analogue inputs but are in general not well supported by the linux software.
However with firewire and ffado is this not a real issue; as long as the interfaces are on the same bus and the clock is synchronised is it possible to stitch them together as one big audio interface. This give the possibility to sample 8 analogue inputs at the same time. More about this in the software part.

Terratec Producer Phase 24FW digital / midi connector pin-out.

I started recently to look into "professional" audio interfaces for Linux. As part of this I have picked up from ebay a Terratec Producer Phase 24 FW in my quest for decent equipment . This is a fairly simple interface with 2 analogue inputs and 4 analogue outputs of which 2 are used for the headpones output at the front of the box.  There are two nice things of this box:

• Fully supported by the FFADO project.
• 24bit resolution with 192kHz sample rate.

There is now-a-days not much information to find about this box. There are some old reviews, but the normal website of Terratec is not showing much anymore. However there is a kind of manual archive with some good information and software. This can be found here. The software is of course MAC and windows only and therefore not so important.

Unfortunately the box came without the adapter cable for the digital connector.  This connector contains S/PDIF signals and MIDI signals in and out.  I thought I was fine; I don't have MIDI equipment and I did not want to experiment with SPIDF. However at the time I didn't realize that the S/PDIF signal is mandatory to synchronize multiple interfaces together on one master clock.  Some audio interfaces can even use a world clock signal, but most use the optical or coax S/PDIF or ADAT signal for this purpose.

The missing digital and MIDI signal cable as shown on the advertisement material of the manufacturer

Therefore It was necessary to find out the pin-out of the digital 9 pin sub-d connector.  It was fairly easy to guess the function of the pins on the connector by looking at the more or less standard circuits for S/PDIF and MIDI interfaces and by some poking around with a multimeter.

Based on the measurements and the standard schematics I was able to get to the following schematic / cable diagram of a part of the inside of the Phase 24 FW box and the resulting breakout cable:

Schematic of the digital / midi connector and the breakout cable

The inside of the Phase 24 FW part is not very accurate but simplified to understand the way the breakout cable should be connected. There are much more components on the PCB than showed in the above schematic. Interestingly not only the Midi-in is isolated, but also the SPDIF-out is more or less floating. This is probably to prevent ground loops. The suppressor diode tries to keep the excessive voltages under control.

The following table gives an quick overview of the pinning of the 9 pin female sub-d connector on the box:

Signal group	Pin	Description
MID out	1	+5V out through 220Ohm
	2	Out signal
	3	Gnd
S/PDIF out	4	Signal out
	5	Suppressor diode to GND
Midi in	6	LED (diode in parallel with LED) through 220Ohm
	7	Return
S/PDIF in	8	82Ohm input
	9	Gnd

Looking at the website and other old data of Terratec I expect that the Phase 22 has the same adapter cable for the digital signals.  And it is very well possible that more products use the same pin out for this 9pin sub-d connector.

Now I have to solder a cable and synchronise my audio interfaces.