Digital Audio & Video

A complete clinic

The September 16 meeting on digital audio was hosted by Mark Parsons of Parsons Audio and the September 25 digital video meeting was hosted by Ross Kaufman of WCVB-TV. The two meetings were organized by Robin Shahid of Boston Post.

Ken Ainsworth of Tektronix gave an excellent presentation on SDI (serial digital issues) and testing as well as the importance of Signal Gamut. Key points of the meeting were:

Typical problems:

1. Intermittent picture "sparkles" or burst errors
2. Equipment that works fine with some sources but not others.
3. Jitter in composite analog output signals.

Inherent SMPTE 259M weak points:

1. No error detection or correction
2. Receiver jitter tolerance not standardized
3. Poor reference for D/A converter clocking

A reliable system must have at least 5 dB S/N margin for each link.

1. Use good quality, double shielded coax
2. Equipment must meet SMPTE 259M jitter spec
3. The number of serial regenerations must be kept small (under 5).
4. D/A conversion must be derived from subcarrier.

Signal Gamut (Gamut is the complete range of anything.)

1. Gamut limits exist in each signal format.
2. The limits are not the same for each format and problems rise when transcoding.
3. Maintaining gamut limits insures repeatable color reproduction.

Ken stressed the importance for monitoring all destination formats to maintain proper gamut and said that a comprehensive jitter specification for SDI will soon be available.