DVL-Digest 614 - Postings: Index Line IN=AGC or Manual? Line IN=AGC or Manual? - "Perry" I'm not sure the levels issue ever got explained, so apologies if this is a repeat. On professional analog equipment (including video recorders) the generally accepted maximum 'peak' recording level was that which gave a distortion of 3%. This is only just noticeable on a well designed system, and the onset of distortion is quite gradual. Setting peaks at this level allows maximum use of the limited dynamic range. On most typical main stream programming, a ratio of 8dB between peak and average levels is appropriate. The average level is normally designated 0dB, and this is usually the level at which line-up tone is sent. On BetacamSP for instance, this 0dB level is about 9-10dB below the 3% distortion point. In Europe, we usually use PPM (peak programme meters) to monitor audio levels; the BBC designates 0dB as tone level with +9dB as the absolute peak for programme sound (nominal +8dB with +1dB allowance). On digital, there is no gradual onset of distortion, it is like hitting a brick wall! On the other hand there is a much wider available dynamic range. On early Digital Betacam machines, Sony set the audio gain such that 0bB tone was recording at -20dB re peak digital level. This gives an extra 12dB of safety zone. Many broadcasters said this was too much, and -18dB for tone is now the generally accepted mark. On consumer DV kit where 12bit gives lower dynamic range, -12dB seems to have become the equivalent figure. This still gives about 3dB safety zone but this would only apply to a very accurately PPM monitored signal. If the programme content has a very high peak to average ratio (typical for uncompressed material) and is monitored using VU meters then the safety zone will soon be consumed. The bottom line is that use of average level tone with digital recorders is not very appropriate and is just a hangover from analog practice. It is far more appropriate to send a tone at guaranteed maximum peak level, but this is not easy to do with a conventional analog mixer board. In practice, even 12 bit digital audio has dynamic range to spare so it is best to always err on the low side and be prepared to turn up the levels in post. Personally I set the levels at the loudest 'rehearsals' I can get with about 6-10dB margin on peaks. Perry Mitchell Video Facilities http://www.perrybits.co.uk/ (diese posts stammen von der DV-L Mailingliste - THX to Adam Wilt and Perry Mitchell :-) [up] |
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