Friday, August 18th, 2017

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

By editorMarch 31, 2012

TECHNOLOGY

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!“In the mid 1990’s Prism Sound undertook a research project – funded entirely by itself – to look into reports that CDs returned from pressing plants did not sound “right” according to the producer or artist.  Such was the depth of feeling about this that huge lawsuits threatened in the music industry between artists and record companies.  SONY music, an interested party, worked with us at Prism Sound, as did Doug Carson of Doug Carson Associates (manufacturers of plant for CD production) to find the answer.

Prism Sound Orpheus Firewire Recording Interface

“Incredibly, our diligent research team headed by Prism Sound’s co-owner Ian Dennis, discovered that the rumours could be true, though probably not for the reasons imagined by the complainants.  It was established that numerically-identical CDs, played on certain CD players, could each produce a different audio output.  The differences were consistent such that it was possible by measurement to identify a specific CD disc from a set, despite the fact that all yielded the same binary audio data stream.  Further, it proved possible to detect if a certain test track was the example from the inner or outer edge of the disc.

“So, those citing sonic differences could be right, even though the digital data was still perfectly intact.”

“These differences turned out to have a perfectly straightforward explanation of course.  In this case, interference from the transport tracking, focusing and rotational speed servos with the DAC voltage reference, vital to the accurate estimation of signal amplitude.  These deficiencies were not present in all players of course, but a consumer purchasing a player would have no hope of determining which sort worked best.

“A great deal of attention was paid to the issue of jitter, or timing instability on the digital clock signals used for A/D and D/D conversion.  Early papers on the subject by the Prism Sound team showed that audio converters often lacked the right defences to protect them from poor quality clock reference signals.  Worse, it was explained that even with good references, a poorly designed clock recovery circuit (usually some kind of Phase-locked loop) could make things worse by having intrinsically poor performance.

“It is often suggested that the most important issue for a modern audio interface is the choice of chips by the manufacturer – as if this is now the determining factor for performance and that the design of the product around the chip set no longer influences performance.  Whilst chip performance has improved a great deal since Prism Sound began its mission in the 1990’s, this could hardly be further from the truth.

“As has always been the case, the electronics around the chips is still the real determinant of performance.  And even much older chip designs, with well-designed supporting electronics, can still be very competitive.  Interestingly, some of the oldest products manufactured by Prism Sound, such as the vintage AD-1 (1993) and DA-1 (1995) are still held in high esteem today by top audio industry professionals and command remarkably strong second-hand prices – still up to 50% of the original list price.

“As an example, PMC (aka the Professional Monitor Company), a leading loudspeaker company with whom we have a close relationship (we operate shared distribution in the United States) recently launched a revolutionary new range of Hi Fi loudspeakers at a spectacular dealer event at the O2 arena in London.  Personal demos were provided to dealers in a nearby hotel suite of the new PMC Twenty series by an enthusiastic Peter Thomas (founder and Chairman of PMC).  The demo setup included a nice CD player, playing though a Prism Sound DA-2 – a product based closely on the original DA-1 and released in 1998.

“Notwithstanding these examples, today’s models can offer even better performance.”

“But, good electronic design still matters.”

“That’s why we still do what we do.”

“Over the past 25 years, not only have we developed a range of audio interfaces that have a world-leading reputation for sound quality, we’ve also developed a range of industrial audio measurement equipment that is used by clients ranging from the titans of manufacturing industry to the geek squad.

“Many of our customers for audio test equipment make products that some might think of as competitive with our own.  However, we don’t really compete with our customers.  Our products are generally a lot more expensive and sell in smaller numbers as a result.

“Why is this?  There are two reasons.

“First, we simply don’t compromise on sound quality.  We don’t work to a production timetable.  We don’t produce a new model every year.  The biggest single cost element in a Prism Sound product is the engineering time spent on design and then refinement, more refinement and more refinement still.  We do it for the art of it.“The second reason is that we make everything locally in the UK.  We recognise that the world demands cheap electronics goods and that China and other low-wage economies represent an optimal environment for low-cost production and this is fine.  We work hard to assist our industry customers and their manufacturing partners in Asia.  But we like to have total control over our own production, for the specific purpose of maximising audio quality rather than reducing production costs.

“In short, that’s why we reckon we make the best sounding converters in the world.”

“In the second of these two articles, my colleague, co-owner and Prism Sound Chief Technical Officer Ian Dennis loses his marbles and accidentally or maybe deliberately reveals to the world some of the secrets of great audio design.  If you are interested in audio electronics design it will seem logical enough when you read it, but you will need to remember that you’ll need dogged persistence and determination to see yourself through the seemingly endless round of testing and refinement that results in truly great sounding products.”

“Thanks for reading and I hope you enjoy Ian’s article!”

Graham Boswell

24-March-2012

www.prismsound.com

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