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Technical Paper 4. SPDIF - an Analogue
Issue that requires an Analogue Solution

Where is mention of Transport + DAC in the Redbook Standard?

When the Compact Disk industry standard called the "Red Book " was conceived, it never crossed the minds of the designers that the transport section and the Digital-to-Analogue section would not be housed under the same chassis and therefore be no more than several centimetres apart.

The Original I2S Connection

i2s
Source: www.tnt-audio.com

The SPDIF Spiv

To transmit the digital signal (which is comprised of many different data streams) between the separate transport and DAC, the "solution" was the Sony/Philips Digital Interconnect Format (SPDIF).

1http://en.wikipedia.org/wiki/Red_Book_%28CD_standard%29
2 Philips (NXP) I2S bus specification 1996
3http://en.wikipedia.org/wiki/Spdif

It is ironic that the common perception is that the SPDIF is a cure-all for digital signal transmission whereas in actual fact, it is more akin to a band-aid.

The reason? It is a little known fact that the SPDIF is actually an analogue transmission system. It was originally developed to transmit a video signal system onto which is "piggybacked" the digital data.

In the late 1970’s, SPDIF was first used when Digital PCM Recording Systems (like the Sony PCM-F1) used the Betamax video tape to store digitally-recorded music.

The Sony PCM-F1

sony_pcm
Source: Sony: mhttp://www.palsite.com/pcmf1ovi.html

This analogue transmission method is subject to the same problems as conventional video systems that use a single video connection for transmission of the colour TV signal (except in case of SPDIF, clocks and signal are mixed together).

Imagine for a moment, the red, blue and green signals from a component video4 signal all carried on the same line!

Rubbing more salt into this open wound, nearly all standard receivers use a circuit technology called a Schmitt Trigger to turn this video signal into something that digital circuits can work with.

4http://en.wikipedia.org/wiki/Component_video

A Schmitt Trigger Circuit

schmitt
Source: hyperphysics.phy-astr.gsu.edu

As a Schmitt Trigger circuit uses positive feedback, it will "kick back" a short pulse into the cable and the SPDIF source (e.g. a Computer or CD transport) at the precise point when it switches. This "kickback" causes distortion of the waveform and is one of the significant sources of jitter in the transmission of the SPDIF signal.

Therefore the problem being purely analogue in nature, cannot be solved by using digital circuitry; it requires an analogue RF engineering solution.

HD-Valve Digital Input (HD-VDi)

At AMR, to overcome this set of analogue problems we use a perfectly logical, analogue solution: the HD-Valve Digital Input (HD-VDi).

A very high frequency valve (a valve that operates to several hundred MHz) in a zero feedback circuit (derived from military radio receiver front-end technology) amplifies the incoming SPDIF signal for the Digital Processor-777’s two "HD" SPDIF inputs.

Following this zero feedback valve amplification of the SPDIF signal, a cutting-edge circuit by AMR restores the clean square wave shape back to the original. Please see the diagram below:

Theory of the DP-777’s HD-VDi

schmitt
Source: AMR

Even with sources that have incorrect output levels or a poor wave form, the input receiver gets a clean, correct level and a virtually perfect source signal to lock onto.

At the same time it also eliminates the commonly seen glitch at the trigger point of the receiver, which sends energy to the source.

In practice, the difference between a typical poor square wave "before" and the almost perfect square wave "after" the HD-VDi section is quite jaw-dropping. Below is a HP/Agilent Oscilloscope depiction of the results which are a real eye opener.

The first graph outlines the typical real-life SPDIF signal that following transmission bear little resemblance to the original square wave. The second graph shows the same SPDIF signal with the HD-VDi engaged. It is clearly visible that the signal has been transformed from a blurred to clean and sharp SPDIF signal.

A Comparison of AMR's HD-VDi in Action

Graph 1: Without HD-VDi:

graph1

graph2
Source: AMR

Thorsten Loesch
Director – Technology
e: thorstenl@amr-audio.co.uk
t: +44 (0) 01704 227 204;

     
 
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