A 2 minute audio demonstration of the GLITS test signal, constructed in Audacity using instruction from this page.
|Problems playing this file? See media help.|
GLITS is an acronym for Graham's Line Identification Tone System, a test signal for stereo systems devised by BBC TV Sound Supervisor and Fellow of the IBS Graham Haines in the mid 1980s. It comprises a 1 kHz tone at 0 dBu (- 18 dBFS) on both channels, with interruptions which identify the channels.
|The left channel is interrupted once for 250 ms every 4 seconds. 250 ms later the right channel has two interruptions of 250 ms spaced by 250 ms.|
This arrangement has an advantage over the EBU stereo ident tone in that each channel is explicitly identified as belonging to a stereo pair. The EBU Technical Document Multichannel Audio Line-up Tone (Tech 3304) defines stereo lineup tone as having an interruption in the left channel only, lasting 250 ms every 3 s.
There is now an official EBU standard for a multichannel BLITS 5.1 channel ident tone which is also described in the Tech 3304 paper, along with an alternative film-style multichannel ident tone system for systems larger than 5.1 arrays.
Blits plays a sequence of tones (based on the musical notes A and E) at -18dBFS on each channel in the AES channel format order (L, R, C, LFE, Ls, Rs), followed by an EBU-style ident on just the front left and right channels, again at -18dBFS and with four interruptions on the left channel. The four interruptions provides a unique confirmation that the stereo or mono downmix came from a 5.1 source and avoids any possible confusion with stereo EBU or GLITS downmixes. The final BLITS tone sequence is a 2 kHz tone at -24dBFS on all six channels – the lower source signal level ensuring that any derived downmixes remain close to -18dBFS.
The alternative EBU multichannel ident tone follows a format more closely associated with the film industry. A sustained 80Hz runs on the LFE channel throughout the sequence. After a 3 second period of constant 1 kHz, -18dBFS tone on all main channels, each channel is identified in turn with a 0.5s pulse of 1 kHz tone, separated from its neighbours by 0.5s silence. The ident sequence starts at Front Left and continues clockwise through each available channel. The amount of time between the 3 second constant tone periods indicates the total number of channels in the system - e.g. a 7.1 system will have an ident sequence lasting 8 seconds.
Snell & Wilcox have used the following on the embedded audio in their VALID8 (Video Audio Line-up & IDentification) equipment:
- Ch1 (L) 980Hz one 250 ms interruption every 4 seconds
- Ch2 (R) 980Hz two 250 ms interruptions every 4 seconds
- Ch3 (C) 432Hz one 250 ms interruption every 4 seconds
- Ch4 (Lfe) 432Hz two 250 ms interruptions every 4 seconds (probably not audible from a subwoofer)
- Ch5 (Ls) 990Hz one 250 ms interruption every 4 seconds
- Ch6 (Rs) 990Hz two 250 ms interruptions every 4 seconds
- Ch7 (Lo) 436Hz one 250 ms interruption every 4 seconds
- Ch8 (Ro) 436Hz two 250 ms interruptions every 4 seconds