Prism Sound : A brief history of time, amplitude … and audio conversion

March 31, 2012


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Graham Boswell

Graham Boswell is Prism Sound’s original founder and current Joint Chief Executive and Director of Sales and Marketing.   He manages the business in partnership with Chief Technology Officer Ian Dennis.  Graham graduated in Electronic Engineering Design and Production from Middlesex Polytechnic (now Middlesex University) in the UK and is an experienced Electronics Engineer and an audio enthusiast.

The inspiration for Prism Sound was Graham’s time at Neve Electronics Labs of Cambridge, UK, where Graham and Ian worked on ground-breaking digital audio mixers during the early and mid-1980’s.  At the end of the second phase of Neve’s development of digital audio mixers, Graham was looking for new challenges and founded Prism Sound as an engineering consultancy, conducting research and development projects for a range of clients including Neve.  The company branched out, consulting for projects in marine radar, optical disk technology and lighting systems but the core expertise remained audio technology.

In 2010 Prism Sound was awarded a Queens Award for International Trade in recognition of sustained business growth over the preceding several years.

Graham continues to talk on the subject of digital audio particularly in the context of measurement and high-end A/D and D/A conversion as part of Prism Sound’s on-going commitment to education delivered through their “Mic to Monitor” and “Mic to Master” seminar tours.

With Graham being in at the beginning of high quality audio conversion, he’s able to tell us much about the history of its development up to the present day.  Here’s what he has to say;

“Buyers of audio interfaces will probably consider the issue of audio quality. For some it may be of passing interest, or less, while for a few it is the overriding factor.  Regardless of which category a buyer belongs to, most will probably sit back and listen at some point to the audio playing from their new interface device and consider whether it sounds any good.

“For a few, this is a matter of critical import. For Prism Sound, the quality of sound reproduction is an obsession.”

“Prism Sound was born in 1987.  These were the early days of digital audio when a 48-track digital mixing console occupied an entire room and consumed more than 5KW of mains power excluding air-conditioning.  Unfortunately, the A/D and D/A converters were relatively poor.  Anti-alias and anti-image filters were largely analogue and rarely maintained consistent performance.  Despite the use of super-computer processing chips, DSP performance was barely adequate.

“Building on the former experience of its founders working on the early Neve DSP (Digital Signal Processing) project, Prism Sound set out to make digital conversion good enough for the most demanding listener.  This presented many problems, not least signal generation and measurement of performance in the digital domain.  During the 1980’s digital audio analyzers were in very short supply and most engineers used analogue test sets combined with digital electronics debugging tools.  In the mid 1980’s the Audio Precision System 1 Dual Domain was introduced, but it was very expensive.

Prism Sound ADA-8XR Converter

“Careful listening was therefore a key part of the regime – this could quickly reveal defects that traditional measurements were ill-suited to detect. One of the obvious digital defects is random but occasional bit errors that manifest as clicks.  Perhaps surprisingly, these can be very difficult to hear, even with sine waves and even more so with musical or other programme content.

“A personal anecdote: I recall a life-changing moment when my respect for audio production professionals was firmly established.  Ted Jensen, then one of the leading new breed of mastering engineers (and still one of the leading proponents today), came to the UK to accept the very first Neve DTC-1 Digital Transfer Console.  Following on from well-received comments about the subtlety and transparency of the digital EQ he remarked that he heard a problem at the low end of the fader range.  We had not noticed anything and had been concentrating our distortion measurements with faders at 0dB or above.  It turned out that Ted heard an arithmetic error in the floating to fixed point conversion taking place at the console output.  This occurred at a fixed bit position (and hence signal level ) and became noticeable at low levels if listening carefully.

“Today we are used to looking at low level behaviour of digital systems quite critically.  Then, we weren’t.”

“The key issues with data conversion between analogue and digital domains are the resolution of time and amplitude.  We need to know a sample value to a high degree of precision at a precise time.  Sounds obvious but these critical factors are often overlooked in real-world products, especially when a designer is on a deadline for the start of production.  Lateness results in lost revenues – and what is the cost of delay for a product that ships 50,000 units per month?  It could cost your job – so why fuss over a marginal extra bit of performance?  Management probably wouldn’t thank you for it anyway!

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