Real World Computing

Brian Heywood looks at cost-effective ways of capturing the sound of music safely onto your computer's hard disk

The quality of the audio you record onto your PC is determined by the weakest link in the audio chain. Even the most powerful computer and best analog-to-digital converter can't compensate for a cheap microphone or a poor preamplifier. The humble microphone has had a somewhat chequered career since the first examples were constructed by Alexander Graeme Bell in the latter part of the 19th century.

His first example of microphone technology was essentially a box filled with carbon granules that had a DC current passed though it. Acoustic energy captured by a diaphragm shook up these carbon grains, alternately squeezing them together and relaxing, which altered the current flow, and that in turn caused a remote earpiece to vibrate in sympathy with the incident sound.

Apart from having a pretty awful frequency response - 300 to 3kHz at best - the carbon grains tended to become compacted, which could be cured only by banging the microphone with gusto on a hard surface. Since then engineers have developed a number of different ways to convert acoustic energy into electricity, including piezoelectric crystals, moving coils (dynamic mikes), ribbon and condenser technologies. It's the last of these technologies, represented by the large-diaphragm condenser, which has become the standard studio microphone (for example, the Neumann U87), an icon of high-quality professional audio recording. This is because the condenser microphone has the best overall frequency response of any of the common microphone technologies.

The way it works is that a thin, but relatively large, flexible metallic membrane has a DC bias voltage applied to give it an electrical charge. Impinging sound waves cause this membrane to vibrate, changing the distance between it and a fixed backing plate and so generating a changing voltage that accurately models the shape of the sound pressure waves. The low mass of the membrane gives it an excellent response to transients and thus a good high-frequency characteristic.

The Phantom Menace

The downside of condenser technology - apart from the cost - is that the microphone needs a DC power supply to maintain the voltage bias on its diaphragm. This voltage is typically 48V, supplied via the microphone lead from the mixing desk, and it's known as phantom power. The presence of a phantom-power facility on a mixing desk indicates that it was designed for professional recording or live PA applications, but it doesn't half complicate system setup if you just want to plug a decent microphone into your PC to record acoustic instruments or voice.

Californian microphone manufacturer Marshall Electronics Inc (no relation to the UK guitar amplifier company) has now come up with a solution: its MXL DRK (Desktop Recording Kit) contains a low-cost, large-diaphragm condenser microphone targeted at the computer music and home-recording market. What makes it suitable for this application is the fact that it uses an internal battery to provide the bias voltage, which means you can plug this microphone straight into your computer, without first having to connect it to a mixing desk with a phantom-power capability.

There's actually nothing new about this idea: my Neumann U87 has this capability so that it can be used for location work. What's novel about the DRK is that it uses a standard 9V PP3 battery to provide the power, rather than the more esoteric and expensive 22.5V batteries required by the Neumann. Couple this with a desk stand and a set of leads designed to handle most PC connection scenarios, and you have a system that you can just 'plug and play' whenever you want to record anything acoustic on your PC.

Continued....