View Full Version : Delta-Sigma & Amplitude

Jan Daugaard
11-02-2003, 03:49 AM
This is not an attempt to settle the discussion PCM vs DSD -- aka DVD-A vs SACD --, but simply a request for some factual information for which I have searched the web in vain.

It would prima facie appear that 1-bit delta-sigma is capable of representing higher amplitudes at low frequencies than at high frequencies: 0 represents a decrease in amplitude, and 1 an increase in amplitude, so at low frequencies we can go on increasing (or decreasing) the amplitude for a longer time than we can at high frequencies. The amplitude that can be represented is thus inversely proportional to the frequency.

However, SACD players can reproduce the same amplitude throughout the frequency range, so there is apparently something I have overlooked.

I'm sure there is a member of this site who knows the answer. A reference to a good primer on DSD would also be appreciated.

Earl K
11-03-2003, 07:45 PM
Hi Jan

I remember about 20 years back when this started hitting the proverbial fan ( within the SR / Studio / Broadcast market).

The best overview ( that I have here ) for these 2 sampling thereoms lies within the printed pages of the "Handbook for Sound Engineers - The New Audio Cyclopedia". The "Delay" chapter written by Mahlon Burkhard ( of IRP - Industrial Research Products ) has the sought after information . This is a "SAMS" book ( # 21983 ) ISBN: 0-672-21983-2 . Quite costly nowadays .

Succintly: ( a quote from Mr. Mahlon Burkhard )

- " Overload in delta modulation occurs when the amplitude of the input signal exceeds the maximum possible amplitude of the reconstructed signal. The maximum possible amplitude of the reconstructed signal is set by the ability of the system to follow the more rapidly varying instantaneous signal values, somewhat like slew rate limiting distortion. This overload limitation decreases as the sampling frequency is increased. It is an inverse function of the signal frequency at high frequencies and is usually expressed as an effective preemphasis. "

I believe the author liked ( preferred this circuit ). Here's an overview of his comparison.

" The following important differences exist between delta modulation and PCM coding:

There is no need for expensive filters with steep slope characteristics - with delta modulation
There is great simplicity in delta modulation coding and decoding, an important consideration when a number of outputs on a single delay line are called for.
There is no sharp limiting of the upper voice spetcrum in delta modulation.
Delta modulation is less sensitive to errors in storage and recovery of the digital signal.
Delta modulation is less suitable for arithmetic manipulation in the digital form. "

One product from that era that I remember used delta modulation was/were delay devices made by "DeltaLab". They did sound very good if not better - with a more natural sounding midrange than comparable PCM coded products. Still, I was already part of the mounting PCM wave (after having bought into Lexicons family of products).

hope this is helpful <. Earl K :)