7 Third-party Audio Feeds
Quite often there will already be audio systems in place for the videographer to take a feed from – such as stage performances where the performers, instruments and, possibly, other recorded feeds are being mixed for venue amplification. It may be possible in these circumstances to take a feed from this mixing desk and input that directly into the camera.
In the world of analogue audio connections, there are a variety of signal levels at which different devices operate. In the early days of audio communications, a “standard level” of 1mW into a 600 ohm line was adopted. The compromise was between higher levels requiring higher power consumption, and lower levels which were more prone to induced and thermal noise. This level was referred to as “zero dBm” (0dB = 1mW/600ohm), so signals could be measured relative to this power level and the results expressed in dBm. In voltage terms, 0dBm = 0.775Vrms (Volts – Root Mean Squared), and this voltage (0dBu) is still a common analogue measurement reference today, along with the slightly different dBv (where 0dBv = 1Vrms).
Broadcast audio equipment still uses 0dBu as its standard for Line Level connections between devices, but a lot of other professional equipment (originally intended for the semi-pro/consumer market) uses a lower Line Level standard of -10dBv. Mic Level, the signal level generated by microphones themselves is generally much lower, in the range –50dBu to –70dBu, but of course dependant on the actual sound pressure levels being converted.
Audio inputs on cameras and other recording devices are normally designated either as ‘Mic Level’ or ‘Line Level’ or are switchable between the two standards. Clearly, with such a big difference between typical Line Level and Mic Level signal levels, it is imperative that you match the input of your camera or recording device to the same level that it is being fed. Any mismatches will probably result in the recorded signal level either being way too high (and distorted) or way too low (and noisy). Furthermore, there are some practical considerations when using external feeds. Firstly, complex audio mixers will have a range of output connections for specific purposes. Auxiliary outputs are generally used to send separate channels to external effects and processing units prior to being sent back to the mixer for final output. These outputs can also, prior to any equalisation, be added to the desk – so generally will not be suitable for feeding to your camera.
If you use this method, consult with the audio technician and ensure you get what is referred to as a ‘Front of House Mix’. It is also preferable to have a Line Level output, ensuring that your camera’s audio input is set to Line Level and not Mic Level.
7.1 Audio Connectors
There are various types of connectors used to link one audio device to another. The most common microphone connector found on professional camera equipment is the XLR-type connector. Non-professional equipment will usually feature 3.5mm mini jack connections, or phonos (for line level connections). XLR connectors are also used for connecting professional VCRs, mixers and other ancillary equipment, along with 1?4” jacks. Where these connectors carry Line Level signals, XLR connections will normally use the 0dBu standard, whereas phonos & 3.5mm mini-jacks usually operate at the lower –10dBv level, and 1?4” jacks may be either.
The main benefit of using XLR connectors, besides providing a very robust contact and securing method, is that they allow for a ‘balanced’ audio connection. Balanced audio cable uses two separate cores for each audio signal, plus an earth screen, with the two “balanced” signal cores carrying versions of the signal
which are out of phase with each other. Any noise or interference picked up by the cables is picked up equally by both signal cores, effectively “in-phase” on both cores. At the destination the signal is “unbalanced” by subtracting the signal voltage on one core from the out-of-phase version on the other, and at the same time cancelling out any added interference noise (which is subtracted from itself).
Using balanced signal feeds allows for the use of long cables, while reducing susceptibility to external noise; especially mains interference.