Feeds

How digital audio ate itself and the music industry

Part One: The birth of a new science

Designing a Defense for Mobile Applications

The numbers game

Sony PCM-1630 adapter

U-matic companion: Sony's PCM-1630 adapter

With digital, go too loud and you’ve run out of road. Excessive peaks can’t be digitally encoded accurately because the A/D converter simply hasn’t enough numbers to do it, and so it just flatlines at the top level. There’s no harmonic pleasantries to be had up there, just a disturbing clipping effect. Yet go too quiet, and you get granulation noise – it’s when the A/D conversion can’t detect significant changes in level and so the binary signal stays at a high or low state for "unnatural" durations which deliver erroneous tones to the quiet passages. Sounds like a problem or two there.

DAT format recording options

DAT's data tracks laid bare (click for a larger image).

Well, the first one is fixed easily: don’t record too loud, or have a limiter (dynamics processor) in tow to flatten any wayward peaks. Going from 16-bit to 24-bit converters also allows significantly more headroom too. The second issue with low-level recording is fixed by introducing a randomising element, so those quiet passages don’t sound weird. And you know what that randomising element is? Noise. Low-level noise, called dither, fixes the problem at the other end of the dynamic range. So don’t ever let anyone tell you digital systems aren’t noisy – digital audio depends on analogue noise patterns to mask the presence of its own artifacts, granulation noise.

As for the sample rates, have you ever wondered why the seemingly arbitrary 44.1kHz and 48kHz were chosen? So, let’s start off with the upper limits of human hearing, say 20kHz. Now let’s double it, because A/D conversion needs to capture the peaks and the troughs of sine waves at this frequency, and let’s add a bit more top end for good measure.

Now, what can we record all this data on the technology available back in the late 1970s? Enter Sony’s trusty U-matic video recorder.

Sony PCM-F1

Before DAT, Sony migrated its U-matic-based
digital recording to Betamax.

Rather than have tape spinning at enormous speeds, you could use a rotary head machine and spray the stereo digital audio data as a multiplexed video signal diagonally onto tape. Both the 44.1kHz and 48kHz sampling frequencies were derived because they were mathematically convenient for both PAL and NTSC U-matic recorders. For early CD mastering, these video tape recorders were fitted with PCM-1610/1630 (pulse code modulation) adaptors and the fact remains that the 44.1kHz sample rate of most of the music we listen to today has its origins in a video recording system that went into production exactly 40 years ago.

Still, the ideas behind the U-matic (and later, Betamax variants) as the CD mastering machine were not without merit and RDAT, in essence, was just a miniaturised version. And whereas the analogue VCR had led the way to provide a storage mechanism for digital recordings, the RDAT recorder evolved into a convenient storage system for back-up and archiving computer data. Sony only discontinued RDAT in 2005. Eighteen years is a pretty good innings for a format.

Dolby B noise reduction

Perhaps more interesting still, are the parallels between the first analogue noise reduction systems from the likes of Dolby and dbx, and later compression techniques in the MP3 era, in use today.

Dolby B type Signetics chip

Dolby B-type Signetics IC from 1973

Back in the day, just about every pre-recorded cassette tape had Dolby B stamped on the cover. The Dolby NR (noise reduction) technology owed a lot to a process called "compansion" – compression on recording and expansion on playback – with a few clever tweaks. Compression meant attenuating the loud bits and leaving the quiet passages unaffected. It has many creative uses, and crucially with analogue recording it enabled quieter sections to be recorded at higher levels (minimising the noise) because the louder sounds were being levelled out automatically, thus avoiding overloading.

Using blade systems to cut costs and sharpen efficiencies

Next page: Sound thinking

More from The Register

next story
BBC goes offline in MASSIVE COCKUP: Stephen Fry partly muzzled
Auntie tight-lipped as major outage rolls on
iPad? More like iFAD: We reveal why Apple fell into IBM's arms
But never fear fanbois, you're still lapping up iPhones, Macs
Nadella: Apps must run on ALL WINDOWS – PCs, slabs and mobes
Phone egg, meet desktop chicken - your mother
HP, Microsoft prove it again: Big Business doesn't create jobs
SMEs get lip service - what they need is dinner at the Club
White? Male? You work in tech? Let us guess ... Twitter? We KNEW it!
Grim diversity numbers dumped alongside Facebook earnings
ITC: Seagate and LSI can infringe Realtek patents because Realtek isn't in the US
Land of the (get off scot) free, when it's a foreign owner
Dude, you're getting a Dell – with BITCOIN: IT giant slurps cryptocash
1. Buy PC with Bitcoin. 2. Mine more coins. 3. Goto step 1
There's NOTHING on TV in Europe – American video DOMINATES
Even France's mega subsidies don't stop US content onslaught
You! Pirate! Stop pirating, or we shall admonish you politely. Repeatedly, if necessary
And we shall go about telling people you smell. No, not really
prev story

Whitepapers

Designing a Defense for Mobile Applications
Learn about the various considerations for defending mobile applications - from the application architecture itself to the myriad testing technologies.
How modern custom applications can spur business growth
Learn how to create, deploy and manage custom applications without consuming or expanding the need for scarce, expensive IT resources.
Reducing security risks from open source software
Follow a few strategies and your organization can gain the full benefits of open source and the cloud without compromising the security of your applications.
Boost IT visibility and business value
How building a great service catalog relieves pressure points and demonstrates the value of IT service management.
Consolidation: the foundation for IT and business transformation
In this whitepaper learn how effective consolidation of IT and business resources can enable multiple, meaningful business benefits.