Topic: Running Schoeps CMD-2U(Digital Microphone Bodies)w/Sound Devices 788T

[blee421]

 I recently purchased a breakout cable from Trew Audio for the 788T.It has 4 digital in's(xlr 110 ohm)and 1 digital out(5&6)which can be switched internally on the 788T.Here is some info from the revised user guide.

 
AES42 Digital Microphones

The 788T is compatible with digital microphones conforming to the AES42 specification, such as the
Schoeps CMD-2U. By setting DIGITAL INPUT: AES42 POWER to Enabled, the 788T supplies +10V
of “digital phantom power” to each of the digital inputs. The gain of the microphone can be adjusted
with the 788T’s front panel gain control just like a normal analog microphone. The gain adjustment is
controlled in the 788T digital domain.
Never apply digital phantom to unbalanced digital inputs, as this can result in damage to the hardware.

The 788T can lock its sample clock to a single AES42 digital microphone signal. This is set from the
Setup Menu option REC: SYNC REFERENCE to the corresponding digital inputs. When the 788T is
set to use its internal clock any connected incoming AES3 or AES42 signal is sample rate converted.
Things to consider when using AES42:
• The 788T supports four AES42 pairs.
• Input Limiters can not be enabled for Digital Inputs.
• The 788T supports AES42 Mode 1 operation, Mode 2 operation is not supported. In other
words, the 788T supports digital audio transfer via AES42. However, it does not support the
AES42 protocol for remote control, microphone identification, or status flag indicators. Mode 2
microphones will automatically operate in Mode 1 when connected to the 788T.
•  When using multiple AES42 microphones set the REC: SYNC REFERENCE to Internal. This
will ensure a synchronous operation between the 788T and digital microphones.


Page 9-user guide

5)  Multi-Function DE-15 Connector
Multi-function DE-15 connector acts as
AES3 Inputs 1-8, AES3 Outputs 5-6, and
Logic In and Out. Analog and digital
inputs can be used simultaneously. See
Connector Pin Assignments.
Sample Rate Converters are enabled
on each AES input when Setup Menu
REC: SYNC SOURCE is set to Internal.
See Sample Rate Converterts.
AES inputs support AES42 Mode 1 oper-
ation, supplies +10 V of digital phantom
power. See AES42 Digital Microphone

page 34

Internal
The 788T disregards any external sync source and sample clocks are internally generated. When set
to Internal, the Sample Rate Converters will be applied to any incoming AES signal.

Sample Rate Converters
The 788T has high-quality, hardware sample rate converters (SRCs) on each of its AES/EBU digital
inputs. The SRCs apply to AES signals when the Setup Menu option REC: SYNC REFERENCE is set
to Internal. The 788T’s digital inputs accept AES signals with sampling rates from 32 kHz to 192 kHz.
With the sample rate converters each digital input can be fed with signals of any sample rate without
having to word clock all of the devices together. 
The SRCs are only active when needed; when not needed they are deactivated. To use digital inputs
without the SRCs active, set the REC: SYNC REFERENCE Word Clock Input, Video Input, or to an
AES input pair.

Info from Sound Devices Support webpage.

AES 42 Support in 788T

Sound Devices introduces AES 42 digital microphone support in the 788T with firmware revision 1.5 beta. AES 42 is a protocol for direct digital interconnection between microphones and devices, such as mixers and recorders.

Why AES 42?
One benefit of AES 42 argued by microphone manufacturers is that by digitizing the signal at the capsule, several analog gain stages are eliminated. Theoretically, conversion at capsule-level improves signal transfer and can result in higher dynamic range. With a digital signal in the microphone, the mic can have on-board digital signal processing (DSP) to perform numerous functions including:  gain control, frequency-domain adjustments such as high-pass filters, and dynamics control such as compression/limiting.

AES 42 and Word Clock
The 788T provides support and connectivity for Mode 1 (asynchronous) connections. Mode 1 uses the microphone’s own clock generator for its word-clock-source. Because the word-clock-signal out of the microphone may differ from the recorder’s set sampling rate, the 788T’s hardware sampling rate converter (SRC) is activated for AES 42 inputs when using the recorder’s word clock. The recorder provides connection for multiple (up to four) AES 42 input sources at any supported sampling rate. The 788T accommodates AES 42 (and AES3) inputs with sampling rates from 32 kHz to 192 kHz.

Connectivity and Gain
The 788T provides “digital phantom power” for AES 42 microphone operation. Similar to a balanced, analog microphone connection, AES 42 requires two-conductors and a shield. An AES 42 microphone signal appears on both “sides” of a connected input pair. For example, when connecting an AES 42 microphone to AES pair 1/2, the microphone signal appears at both input 1 and input 2.

When an AES 42 microphone is connected to the 788T, the recorder’s front panel gain controls digital gain levels in the recorder, not digital gain in the microphone and not analog levels.

Unsupported AES42 Features
The AES42 protocol includes several modes of microphone control and connection. The 788T presently does not support the following:

Mode 2, the 788T does not send word clock signals to AES42 microphones nor does it send control signals to control…
in-microphone digital gain, gain is controlled after the microphone’s digital gain stage,
DSP control, the 788T does not control AES42 DSP processing.
Is AES 42 Relevant?
As AES 42 hardware becomes available from manufacturers such as Neumann, Schoeps, and Sennheiser, Sound Devices will evaluate its application in mixers and recorders. The identified benefits of AES 42 appear real for some applications. Whether they prove to be a demonstrable for field production remains to be seen.



So if I got 4 Schoeps CMD-2U bodies,one pair for the MK41's>KC-5 and another pair for the MK22's>KC-5 this would work for a 4 channel recording?I also read that while the digital mics are running I would still be able to run all 8 analog channels as well?

Here is a link to the revised user guide:  http://www.sounddevices.com/download/guides/788t_en.pdf
Schoeps link to CMD-2U:  http://www.schoeps.de/E-2004/cmd2.html
Sound Devices AES 42 support: http://www.sounddevices.com/notes/recorders/788/aes-42-support-in-788t/

[F.O.Bean]

WHOA! AMAZING to say the least. I wonder if Schoeps made the digital bodies to be run w/ the 788T Brian Wonder if Schoeps did a lot of its bench testing w/ a 788T

[DSatz]

Bean, when Schoeps designed the CMD 2 there weren't any recorders available for it. That particular egg came before the chicken. The first product on the market that the mikes could actually be plugged into and used with was a mixer that, as far as I know, isn't distributed in North America.

Before the CMD 2, there was a two-channel prototype "digital bottle" preamp/converter which Schoeps showed at a couple of AES conventions and the like. I saw it, and seem to recall that you could plug one of their CMXY 4V analog stereo microphones directly into its input socket, but I could be tripping. At any rate, it was designed before any other equipment existed that it could be used with.

Fortunately the engineer in charge of digital development at Schoeps, Christian Langen, was also the principal author of the AES-42 standard. Also fortunately, there is both a great respect for industry standards and there are collegial relationships among the engineers at the various German microphone manufacturers. They generally make a sincere effort to create products which will work properly together. As a result, if I recall correctly, the second product which would work with the Schoeps CMD 2 was the Neumann DMI-2 digital microphone interface.

--best regards

Edited nearly a year later to correct: The Neumann DMI-2 interface can work with a single Schoeps CMD 2 but not with a pair of them unless the recording device has sampling rate converters built in. Schoeps' digital microphones support AES 42 Mode 1 but not Mode 2. Sorry about the mixup.

[John Willett]

I have been running digital microphones for over two years now and have recorded four CDs with them (Neumann KM-D series). 

The Schoeps and SD 788T are only Mode-1 operation, which means that they have to go through a sample-rate converter to clock them.

Luckily the 788T has a built-in sample-rate converter.

Though personally I prefer the Nagra VI over the 788T.

[boojum]

Wouldn't it be a good idea to call SD and ask them straight up?  Color me silly, but that is what I would do.   

[baustin]

This whole digital microphone thing confuses the hell out of me...

Do they have a built in A/D converter? Do they have selectable bit-depth/frequencies? Sorry... I haven't even begun to read about them. Advantages?

[ghellquist]

Do they have a built in A/D converter? Do they have selectable bit-depth/frequencies? Sorry... I haven't even begun to read about them. Advantages?

Yes. Built-in A/D. Close as possible to the source and especially tailored to work there.
Selectable frequencys yes.
Mics can be synched to an external clock (the Mode 2 operation, alas not supported by 788T).
You can expect extremely good mic pre + conversion of the clean type. No way to influence the sound at the source though (no Neve preamps and such although the mic could have limiter/compressor and such).
Paves the way for mic + bit bucket as all you need to carry.

Disadvantages - only on the expensive side of the spectrum at the moment, both for mics and for recorders.

// Gunnar

[DSatz]

baustin, a "digital" microphone is simply an analog microphone with an a/d converter built in. Whether the sampling rate and bit depth are adjustable or not is up to the manufacturer; some are and some aren't.

The AES42 standard, which all present-day "digital" microphones follow, was written so that standard XLR-3 connectors and cables could still be used (for short to moderate cable lengths, anyway). As a result, there aren't enough wires for a balanced signal plus powering plus shield plus a clock signal from the receiving device. So each microphone always runs on its own internal clock.

This makes for a problem in two-channel or multi-channel recording: The sampling rates of the microphones are never perfectly identical, nor do they start sending their data frames at the same time. Thus there's no simple way for the receiving device to interleave and combine the separate signals from two microphones into one digital bitstream.

Some AES42 microphones support a mode of operation in which the microphones accept control signals (though not a clock, strictly speaking) from the receiving device, ingeniously embedded within the powering, so that they soon get "nudged" into sync with each other.

Alternatively, some AES42 recording devices accept the existing, slightly out-of-sync inputs from two or more microphones, then convert the sampling rates of the signals to a common external reference; the recomputed bitstreams can then be recorded or mixed.

Those are the two basic ways to record stereo with digital microphones. Either way requires some fairly complex, and unfortunately somewhat expensive, hardware at the receiving end for the signals.

--best regards

[jobseek2001]

Thanks, but:
Digital gain? 
Clock in the mic and not in the recorder?
If this is a reference design for this standard I see room for improvement(s).

[ghellquist]

Digital gain? 
Clock in the mic and not in the recorder?
If this is a reference design for this standard I see room for improvement(s).

Not quite certain what you mean. But the standard is defined since long time, not expected to change. The 788T is not any reference implementation.

Digital gain -- gain control might not really be needed. An AD might have 20 bits of true resolution or slightly more. So if you set the noise floor on or about 20dB equivalet SPL clipping will end up at about 140dB. Plenty of place for many recording situations. Still, the mode 2 allows you to control gain inside the mic remotely. A mic manufacturer might elect to have a switch on the body (gain control) instead. And there are clever constructions that can improve the "range" of the AD-process above 20 bits. Given 24 true bits it is possible to have the digital noise floor at about 0dB SPL and have clipping then at 140dB.

The clock synching issue may be less important than expected. Low-priced IC-circuits are already available. A tiny signal degradation can be expected of course, but we really are talking tiny. Probably less than the effect on an analog signal going down a longish mic cable.

One thing history has taught is that digital electronics goes down in price over time. Good analog electronics rather seems to increase in price. So keeping all analog circuits inside the mic body will probably lead to lower total system costs over time. (Reasoning: the mic requires analog electronics anyway, so why not put it all there. No need then for any other part to be analog ).

And, of course the transition will take time. And will not be a perfect fit for everyone - some people even today preferr to use analog tape. People will still use their old cherished mics and preamps for a long time to come.

// Gunnar

[jobseek2001]

Digital gain? 
Clock in the mic and not in the recorder?
If this is a reference design for this standard I see room for improvement(s).

Not quite certain what you mean.

Well, why stick with the old cables when you invent a new standard?
Why limit yourself to compromises?
It doesn't cost that much to provide a clock from the deck.
It doesn't cost that much to enable some type of analog gain control (i2c?) by the deck over the mic.

Using the described implementation you have invested quite a lot of $$$ and gotten a rather poor implementation.
Yes, I don't mean to say that it won't perform but I mean the technical choices and logicalities.
(or whatever, I am not a native speaker)

Over three wires you can provide power, clock and receive audio data. Add more wires if you need balanced signals, longer distances, etc.

[boojum]

Over three wires you can provide power, clock and receive audio data. Add more wires if you need balanced signals, longer distances, etc.

I think the idea was to keep it to three wires so that the current XLR connectors would still be the standard. 

[ghellquist]

But there is clock control and analog gain control. And control over any filters built into the mic. Even if there is compressors or other signal handling in the mic there is control over that. The standard covers all this and leaves lots of room for other functions in the mic. The Neumann mics supports some of these functions today.

Check this screen dump of the Neumann control application.
http://www.neumann.com/zoom.php?zoomimg=img/Linkgraphics/RCS_1Ch_Menu.JPG&zoomlabel=RCS&w=128&h=715

Check this list of remotely controlled functions for the D01
- Polar pattern5)
- Low-cut: flat, 40, 80, 160 Hz
- Pre-attenuation: 0 , -6, -12, -18 dB
- Gain: 0...63 dB in 1 dB steps, clickless
- Testsignal: 1 kHz, pink noise, white noise
- Parametric compressor/limiter (incl. de-esser function)
- Independent peak limiter avoiding any clipping
- Switch functions: soft muting, phase reverse, signal lights
- Signal lights: red5) and blue LEDs (switchable via RCS or User Port)

http://www.neumann.com/?lang=en&id=current_microphones&cid=d01_description

The clock control does not work quite as we are used to but it gives the expected function.

But as has been mentioned, the 788T does not support the mode 2 due to design decisions.

The great thing about using stander XLR cables is that the infra-structure is already there. Think about all the miles and thousands of XLR-s already installed in studios and concert halls all over the world.

// Gunnar

[jobseek2001]

But there is clock control and analog gain control. And control over any filters built into the mic. Even if there is compressors or other signal handling in the mic there is control over that. The standard covers all this and leaves lots of room for other functions in the mic. The Neumann mics supports some of these functions today.

There is?
Great!
That wasn't clear from the earlier posts.
How is that implemented with just three wires? Any links to details of the implementation?

[ghellquist]

I believe you need to pay somehow for the actual standard definition (beeing member of AES or similar). Not much money for a company, a bit much for me.

A quick google on AES42 Standard gives quite a few hits, one of the more detailed technical is this one:
forums.futura-sciences.com/attachments/a/19785d1176392516-alim-fantome-micro-numerique-aes42-2006-c.pdf

// Gunnar