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Old 8th September 2014, 11:08 AM   #1
tuck1s is offline tuck1s  United Kingdom
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Default Suggestions please for 16-channel 24-bit digital audio recorder

I would like to interface one of the Behringer: POWERPLAY 16 P16-I

with an ARM based single board computer with an SPI input (pretty much all of them do - from the Raspberry Pi to newer, more powerful ones).

The "Ultranet" output is apparently just 2x AES3 data streams over Cat5 cable (running 8 chans x 48kHz sampling rate on each stream): X32 User Net • View topic - The pm16-m protocol

So they are using 2x AK4114 digital interfaces, which seem to be obsolete (there is an AK4118 which maybe is a successor part, but seems hard to find).

Transformer isolation is used in this system, so thought I should do the same. Given the Cat5 interconnects used, the architecture I have in mind goes like this:

RJ45 "PoE" compatible MagJack --> 2x clock recovery --> 2x SPI inputs on ARM board

the MagJack could be something like:
SI-52008-F - STEWART CONNECTOR - SOCKET, 1 PORT POE GIGABIT MAGJACK | CPC

I would appreciate any comments, suggestions. Particularly if there are projects already going on that I could re-use, e.g. for the clock recovery.
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Old 8th September 2014, 05:05 PM   #2
tuck1s is offline tuck1s  United Kingdom
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.. just to add, I plan to try with the following interface module. The WM8804 seems to be a well regarded device for clock recovery.

WM8804 S/PDIF TOSLINK to IIS Converter I2S Codec Project integration DIY | eBay
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Old 12th September 2014, 06:01 PM   #3
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There are various "digital snake" protocol existing in pro audio industry. One onf them was AutioTrail (AudioRail Technologies.). Anyway, it's not clear how the Behringer protocol works, I mean how they mark the frames in AES3 stream. AKM datasheets though suggest using the hardcoded preamble for non-PCM encoded audio bitstreams.

Anyway, SuperMAC data is "scrambled", DSP of FPGA is needed to convert data to TDM (SuperMAC internal format) or multiple I2S streams. Protocol also includes some error correction, and the cable also delivers the clock signals (so I assume these are used for framing).

SuperMAC | HyperMAC

Last edited by mhelin; 12th September 2014 at 06:03 PM.
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Old 12th September 2014, 08:45 PM   #4
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Seems Ultranet is some simpler protocol than SuperMAC. Something like what's proposed for the AES-X196 (though that standard development seems to be practically died). Might indeed be possible to find out that.
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Old 12th September 2014, 09:34 PM   #5
tuck1s is offline tuck1s  United Kingdom
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Quote:
Originally Posted by mhelin View Post
Seems Ultranet is some simpler protocol than SuperMAC. Something like what's proposed for the AES-X196 (though that standard development seems to be practically died). Might indeed be possible to find out that.
Thanks for your comments .. I've measured the signals coming out of the cat5 ports and confirmed some of the details already known here:

X32 User Net • View topic - The pm16-m protocol

Reproduced here for ease:

Pin configuration
Looking at the back panel of the x32 and counting 1-8 (left to right) we have

1: Ch 1-8
2: Ch 1-8
3: Ch 9-16
4: Vcc
5: Vcc
6: Ch 9-16
7: GND
8: GND


Vcc is 15 volts DC (pins chosen are similar to Power over Ethernet standard). This comes from an internal switch-mode PSU and is intended to power the P16-m external monitoring boxes. It's not stated how much current each connector is specified to source (there are six output connectors on the P16-i). The P16-m's can also be powered by an external PSU, and the back panel reads "12v 300mA". So it's probably safe to say that each P16-i output socket could provide around 4 watts. To be on the safe side, I'll not use the PoE power as it's probably not enough for a recording device with a hard drive.

One other thing to note, is that the P16m requires some power draw on the Vcc pins, to switch on the data outputs. Some experimentation found that 5k6 resistor from Vcc to GND does NOT switch it on; whereas a 2k2 resistor DOES switch it on. So the P16m is looking for approx 7mA current draw to switch on the outputs.

Each output (1, 2, 3, 6) is fed via half of a 74LVC245 octal buffer transceiver device, used in output-only mode.

twisted-pair cable is meant to have a characteristic impedance of around 110 ohms. I've terminating output 1-2 via a Magjack (i.e. transformer isolation) into 110 ohms. There I see approx 500mV pk-pk digital waveform. It looks fairly clean and nice.
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Old 13th September 2014, 10:43 AM   #6
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Ok, fine. WM8804 may not be right part for the AES3 though. You would be better using CS8416.

Using a level converter you might want to first record (directly from the MagJact output using a simple resistor level converter with a series capacitor) the data to PC soundcard with bit perfect S/PDIF input supporting 24bit/192kz data to see if it's just pure PCM data (vs. non-PCM). It is possible that the framing information is embedded into audio data LSB and could be easily reverse engineered.

Last edited by mhelin; 13th September 2014 at 11:11 AM.
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Old 13th September 2014, 11:10 AM   #7
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Quote:
Originally Posted by tuck1s View Post
So it's probably safe to say that each P16-i output socket could provide around 4 watts. To be on the safe side, I'll not use the PoE power as it's probably not enough for a recording device with a hard drive.
24 bits*48000 smp/s*16 channels = 2.304 MB/s which means you should be able to store the data to Class 4 SD cards (4 MB/s). You could use some of the newest Cortex-M4 CPU's so the 4 watts would be more than enough to power a simple recorder.
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Old 13th September 2014, 11:39 AM   #8
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One thing to note: the Behringer specifications for the P16 system tell:

Dynamic range : typical 92 dB

So it might be possible that the data is packed to for an example 24bit/96kHz frames, and possibly using DTS or DD decoding with real 16 bit DR (72 dB SNR + 20 dB headroom).
That is perfectly fine for monitoring application though. Might be one reason why they used the AK4114 part as it recognizes DTS automatically.

To see if that is a case please try connecting P16-I to you AVR (using level conversion to S/DPIF of course).

Last edited by mhelin; 13th September 2014 at 11:52 AM.
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Old 13th September 2014, 10:14 PM   #9
tuck1s is offline tuck1s  United Kingdom
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Hi, thanks for your comments. According to the work done by roblof on the x32 user forum, the data format is simply 8 channels x 24 bits x 48k on each TD+- pair, encoded as a single 24/192 stereo interleaved stream. I have not got that far in verifying this myself yet.

Re. CS4816 vs WM8804 - (briefly) what aspects of the Cirrus part are more suitable for AES3? Is it a data format issue?

Do you know of a good source of prototype / dev board with the CS4816 (to reduce the need for tiny surface-mount-package soldering ..)
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Old 14th September 2014, 10:57 AM   #10
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Quote:
Originally Posted by tuck1s View Post
Hi, thanks for your comments. According to the work done by roblof on the x32 user forum, the data format is simply 8 channels x 24 bits x 48k on each TD+- pair, encoded as a single 24/192 stereo interleaved stream. I have not got that far in verifying this myself yet.

Re. CS4816 vs WM8804 - (briefly) what aspects of the Cirrus part are more suitable for AES3? Is it a data format issue?

Do you know of a good source of prototype / dev board with the CS4816 (to reduce the need for tiny surface-mount-package soldering ..)
I don't think roblof has verified anything. He is just guessing (" Could it be adat compatible?" - actually AES3/SPDIF and ADAT use incompatible coding systems, and AK4114 can handle the ADATA one). Problem is that of framing. You can obviously send 4x 48kHz data in 192kHz stream, but how do you know which channel is which one? AES3 is just a stereo channel. Dolby Digital (AC-3) instead packs 6 channel inside single stereo channel meaning it uses SPDIF framing. Ultranet might use something similar compression and packing into stereo frame system. That's why the lowish 92 dB dynamic range instead of that of converters (114 dB for the Crystal converters).

Btw. what are the main components on P16-I?

Last edited by mhelin; 14th September 2014 at 11:17 AM.
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