Test and Measurement News

Time saving at TANDBERG Television: Testing signal paths with delay

Automating measurement of an electronic device is easy when the signal you put in comes straight out of the other end.  When it comes out some time later, it's a bit more difficult. How can you make a frequency response measurement if you don’t know when the tone you put into the device will come back out? The delay could be a couple of seconds or even minutes, hours or days.

Bob Shackleton of TANDBERG Television, the world’s leading provider of live, on-demand and interactive TV solutions, is a Prism Sound dScope Series III user who wanted to find a speedier method for performing tests of signal paths with delay.

Testing signal paths with codecs and delay
In particular, Bob wanted to perform sweeps of amplitude and THD+N at fine, linear frequency steps because his transmission path involved one or more audio compression codecs.

One solution is to measure the delay, send out each frequency and wait for the delay period before making the measurement.  This is fine where the delay is small.   What do you do where the delay is long or you don't know how long the delay is?

Whatever you do, you can be sure that measuring a lot of points on a sweep will take a long time... longer than the number of points in your sweep multiplied by the delay in the system.

Bob Shackleton called in the Prism Sound applications team to help.  As purchasers of the Prism Sound dScope Series III audio test system, TANDBERG Television benefits from free engineering support.

Prism Sound’s applications team leader, Ian Heaton, recommended a solution to this problem using the dScope Series III to generate a stepped sweep at a rate that its analyzer can track using a "sensed sweep".

Making sense of delayed sweeps
A “sensed sweep” works by automatically detecting a change in frequency or amplitude at the analyzer input and then plotting the next point in the sweep.  This is commonly used for external sweep sources such as DVD & CD players where the sweep is recorded on disc.

In this case, with a delay in the system, we then don't need to worry about when the generated signal would be received.  Instead, the dScope sends out the sweep as a preset series of tones one after another and then simply waits until it detects them at the analyzer input.

It performs the measurements when they arrive, tracking the frequency (and/or amplitude) changes and checking that the same number of tones/sweep points that were sent out have arrived back again.  

The time taken to perform the sweep is now simply the time taken to perform the sweep plus the delay time in the system.  With sweeps with a large number of points and a delay measured in seconds, this can mean time savings of minutes (or hours …) per sweep.

A dScope Series III script file which automates this process is available here (zip file, 5kB with notes)

For further information about this or any other dScope series III applications, contact: scripting@prismsound.com . Important: please include the word "dScope" in the subject line of any correspondence to this email address as we get a lot of spam and it will help it to get through the filters.