AES 42 Support in 788T

Sound Devices introduced 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.