It may help to differentiate this peak level track from other, safer, test tracks by using a different tone frequency, or with different frequencies on each channel.It might be about the balance of a mix or the sound of an instrument, it might be about a particular piece of equipment or signal processing, or it might be about a monitoring system or listening environment.
Whatever the case, an essential prerequisite is to have a valid frame of reference something upon which you can base a subjective opinion. For example, one of the most widely available is Alan Parsons Sound Check CD, but others include the Canford Audio Quick Check CD (theres more about these first two elsewhere in this article), and the three-volume set of Test Discs produced by Hi-Fi News magazine. Other reference discs which are useful (but much harder to track down) are the BBC Radio Group 2 Acoustics Test CD and the EBU SQAM (Sound Quality Assessment Material) disc. If you can get hold of the original Hi-fi News Record Review test CDs, now discontinued, these are an invaluable resource. Useful though these commercial discs certainly are, the range of technical test tracks is often inappropriate for the majority of needs and the selection of musical material rarely matches personal preferences. It can also be used to help assess the acoustic characteristics of the listening environment, and to provide comparison tracks with which to judge a mix or the validity of a mastering process. However, the basic ingredients will be largely the same for all some technical test tracks, some solo instrumental and vocal tracks, and some commercial music tracks its just a case of deciding the specific roles in which the reference disc has to satisfy. This involves being able to check and set such parameters as identification of the left and right channels of a stereo system, the relative phase of two channels, the standard operating level, the amount of headroom, and the frequency response. The specific test signals required by different users and systems may differ, so Ill list those which I have found useful enough to include on my own reference disc (which has evolved gradually over many years). It is a test signal known as GLITS, which was developed by a BBC TV Sound Supervisor called Graham Haines in the mid 1980s. GLITS is an acronym for Grahams Line Identification Tone for Stereo, and it comprises a 1kHz sine wave tone normally at a level of -18dBFS. If you use a 440Hz tone frequency, it can serve as a tuning aid as well) The left and right channels carry an identical level tone which is in the same phase on both channels. The BBC Radio Group 2 Acoustics Test CD is invaluable, but unfortunately, it is now hard to find. For example, those working with analogue desks and standard VU meters may prefer to use a test level of -14dBFS which equates to 4dBu in the European environment and therefore zero on the VU meter. It might be worth reminding Mackie console users at this stage that the meters on those desks are calibrated for 0VU 0dBu, not 4dBu.) Whatever the choice of line up level, it is vital that you document precisely whats on the disc in the sleeve notes so that anyone else using your disc knows what to expect. The length of each mute and the repetition rate arent critical: there simply needs to be enough continuous tone between mutes to allow a reasonable chance to adjust gain controls, and the mutes themselves have to be large enough to enable a meter with a slow fall-back ballistic to react sufficiently to be easily observable. ![]() ![]() If everything is correct then a constant tone will be heard with three 6dB dips in level. Audio Test Cd Full Level PulsesHowever, if one channel is of opposite polarity then the continuous tone periods will become mute (since the left channel signal will cancel the right), and the interruption periods will become three full level pulses instead. All in all, this is a quick, simple, and extremely useful test signal which is absolutely perfect for establishing the signal routing and level calibration of consoles, record machines and effects units, as well as checking metering and monitoring systems. For that reason, I also include a simple spoken voice test track which consists of a clean recording of someone saying In phase. When playing the track back, if your ears agree with the commentary then all is well. However, to avoid potential embarrassment or the destruction of the loudspeaker drivers, I prefer to use a test signal which starts off at the reference operating level and builds slowly to peak level.
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