DG1200 Tally & GPI Interface
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The Problem

Today’s production switchers are frequently equipped with internal digital video effects processors (DVEs or transform engines) to produce “over the shoulder”, “double box” and other multiple source composited effects. Depending on the number of video transforms in use, the video delays through the production switcher are usually predictable and typically in whole frame increments.

Any time that one or more of the DVE processors are on-air, the associated video sources will be delayed, resulting in a lip sync error. By itself, this lip sync error may or may not be noticeable and/or objectionable. However, when cascaded with other lip sync errors that typically accumulate from the original acquisition point to the viewer, the end result can often be objectionable.

The Solution

The video delay through the switcher is usually predictable based upon the configuration for any given combination of effects. Therefore, GPI and/or Tally outputs can be programmed in the effects memory of the switcher and fed to the DG1200. The DG1200 interprets the GPIs and Tally signals and generates the appropriate delay commands to steer the audio synchronizer(s) and automatically eliminate the lip sync errors.

The DG1200 has twelve input channels, each consisting of a GPI Start pulse, a GPI Stop pulse and a Tally line. Each input channel also has a linked delay time register with a user selectable value from 20 µsec (nominally zero delay) up to 6.5 seconds, in increments of 100 µsec. Delay times can be entered and displayed in milliseconds or in TV fields (NTSC or PAL). Input channels can be configured to respond to Tally only, GPIs only, or Tally gated by GPIs for maximum immunity to false delay insertion.

Any input channel and its time value can be routed to any of the five output timers and each timer can steer a separate AD3100 Audio Synchronizer. The output timers can have different time values and can be turned on and off independently. Any timer can be controlled by more than one input. Let’s say that one switcher effect needs a 1 frame audio delay and another effect needs a 2 frame audio delay. Input #1 (or any other input) can enable a 1 frame delay in Timer #3 (or any other timer) and the associated AD3100. Any other input can be used to enable a 2 frame delay in the same timer.

Pre-Delayed Audio Application

The most comprehensive solution is to add AD3100 Audio Synchronizers ahead of the audio mixer. This configuration ensures that all sources contributing to the program output have the correct lip sync.

For applications that require more than 5 audio inputs to be delayed, this solution is scaleable with additional DG1200s and AD-3100s.

Post-Delayed Audio Application

In this simpler configuration, a single AD3100 Audio Synchronizer is added at the output of the Audio Mixer. The amount of delay added to the audio path is chosen as a compromise for the sources contributing to the program output in any given effect.

For example, in a typical newscast over the shoulder shot, the studio anchor has zero video delay and the remote reporter (in the box) has 1 frame of video delay. Setting the AD-3100 delay to between 0.5 and 1 frame is the best compromise for both sources. The studio anchor’s audio will be slightly late and the remote reporter’s audio slightly early. Splitting the difference and choosing 0.5 frame delay is generally not the best choice since the early audio of the remote reporter is more noticeable than the delayed audio of the studio anchor.

The residual lip sync errors are significantly reduced compared to doing nothing at all.

Rapid Delay Change With Pitch Correction

The video delay of the DVE may be switched in and out of the program path several times in a relatively short time. Therefore, it is essential that the audio delay “catch up” quickly. The AD3100 incorporates automatic pitch correction to allow rapid delay change without introducing undesirable artifacts such as pitch shifts, clicks and pops in the output.

Conventional audio synchronizers typically limit the rate of change of delay to around 0.5%. This means that for a 1 frame video delay change at the beginning of a program segment, the audio does not “catch up” until almost 10 seconds later. And another 10 second “catch up” period occurs at the end of the segment when the video delay reverts to normal.

The AD3100 has an adjustable rate of delay change of up to 25%. So, in our example of a one frame change in the video delay, the AD3100 will “catch up” in just a few frames – well before the viewer will notice.


12 input channels each consisting of: Tally Input; Tally Enable On (GPI trigger); Tally Enable Off (GPI Trigger). Each input can be selected for High=True or Low=True. Each input channel can be set to operate with Tally only, GPI Start and Stop triggers only, or Tally gated by GPI Start and Stop triggers. Any input channel can be selected to control any timer output. All inputs are on terminal strips.

5 independent timers provide TTL level steering pulses on BNC connectors (75O source impedance) to control the delay of compatible audio synchronizers such as the AD3100. The range of delay is from 20 µsec (nominally zero delay) up to 6.5 seconds in 100 µsec increments. Values can be entered and displayed in seconds, NTSC fields or PAL fields.

External wall mount supply 110-120 VAC, 60 Hz
or 220-240 VAC, 50 Hz
Consumption 25W at 12 VDC

Dimensions 19” x 1.75” x 9.2” (WHD)
(48.3 cm x 4.5 cm x 23.4 cm)
Weight 3.5 lb (1.6 kg)


Operating temperature 0deg C to 45oC
Storage temperature -25oC to 75oC
Humidity 10% to 95%, non-condensing

Pat. 7,333,150

(C) 2004 by Pixel Instruments Corporation with all rights reserved

Pixel Instruments continuing product development programs may result in changes to specifications and features without notice.