9th November 1999 - Digitally interfaced microphones

Steve treated a packed room to an insight into the theory, philosophy and design of Digitally Interfaced Microphones, and also into the operation of SC 04 04, in its working towards a standard in this field. Starting with the historical reasons for the involvement of Crystal, with their A/D and D/A converter chips and their digital interface chips, Steve moved on to ask why anyone should want a digitally interfaced mic. Considering the esteem that many analogue microphones have attained after decades of development it may seem strange, however there may be several very strong advantages: the ability to place the A/D converter as close to the capsule as is possible, reduces the susceptibility to analogue interference; the traditional problems involved with cables, especially long runs, such as capacitance and again interference become irrelevant; multi capsule microphones can have their polar responses modified by DSP, rather than simple mixing; this DSP could also emulate ‘preferred’ older and fragile analogue microphones; and in addition each mic can be given its own unique 64 bit identification code; of course it is an obvious candidate for connection to a digital desk.

Steve discussed the problems that are thrown up by the introduction of such a product, and therefore the requirement for standards. There are the issues of synchronising the clock of the A/D converter, the need for power for the microphones greater than we are currently used to with phantom powering, and consequently the possible requirement for a replacement to the ‘traditional’ 3-pin XLR. In turn this also calls for a close involvement between manufacturers of microphones and mixing consoles. The clock has to remain as close as possible to the A/D converter in order to keep jitter to a minimum, this can be simply solved utilising an asynchronous clock: this is known as Mode 1. This will require sample rate conversion before interfacing with the desk. A solution to this is the use of a dedicated ‘stage box’ that can not only contain many microphone channels, but also could contain other facilities such as talkback. Drawbacks of this system relate to the poor phase response that will be a consequence of the sample rate conversion.

Another approach would be to use a synchronous clock, however this is likely to suffer from worse jitter than Mode 1 unless it is carefully engineered. Obviously this requires a clock source which needs to be conveyed to the microphone. This synchronous version is known as Mode 2, and comes in three versions, 2a, 2b and 2c. 2a utilises two twisted pairs, one for clock and one for signal: this is awkward and incompatible with other studio cables. 2b uses a hybrid approach with clock data sent as a phantom circuit and power also, giving three logic levels and requiring a close impedance matching. 2c involves sending low data rates embedded within the power supply to control the synchronisation, this suffers from poor jitter and phase response. At this point Steve highlighted the possible demands for a new interface format, this could overcome the problems (and dangers) of power distribution, encompass the requirement of clock distribution and reduce the risk of inadvertently misplugging the analogue and digital desk inputs. Various alternatives have been suggested, some to incorporate additional pairs for synchronisation (possibly a new 5-pin variant to remain compatible with the 3-pin XLR), the RJ45 plug as they are cheap and reliable although not that robust, or something completely different to avoid confusion. The most likely solution however is a modification of the current XLR incorporating an insert that could keep analogue and digital leads apart, yet allow insertion into desks that could discern between analogue and digital microphones. There is a separate task group investigating the connector issues which is yet to be resolved; Mode 2b is looking likely to be the best choice however, for the synchronous digital microphone.

The first digital microphones are starting to become available, for now these are Mode 1 (for which the Standards Committee is close to the ‘Call for Comment’ stage); the testing of Mode 2 is still continuing. Steve wound up the proceedings by answering a few clarifying questions, and by offering an open invitation to join the AES Digitally Interfaced Standard Group by emailing SC_04_04_D@aessc.aes.org.

Crispin Murray