Saturday, 13 February 2010

Video and Audio

I recently did a Pro Tools 101 course. Audio is a little outside my remit as a pixel wizard but I've been a musician for a while and have recorded my own music and obviously I have to deal with audio when editing so it wasn't a completely alien territory for me. What struck me is the amount of similarities between digital audio and video, both technically and in the way you work with it.

Sample Rate
This describes the number of times an analog signal (ie an audible noise or an analog synth) is sampled (or analysed) to produce the digital signal that is recorded. Analog audio is continuous and infinite in its amount of data, but digital audio has to have a finite amount of information and so it cannot be an continuous stream, instead it is a series of samples or records of what the analog signal was doing at a specific moment in time. Typical sample rates are 44.1Khz (or 44,100 samples a second) for music and 48Khz for sound for Film and TV. They can also be higher than this for specialist applications like sound design, but you'll find that these are the most common. Initially I equated sample rate to frame rate. Light is continuous and in order to capture it we take a frame at specific moments in time (interestingly you have to do this even with the "analog" medium of film). Compared to 48,000 times a second, 24/25/29.97 times a second looks pretty insignificant. But I don't think we can equate sample rate to frame rate. If you heard a sample, it wouldn't even register, and in order to recognise a sound you'd probably have to hear at least 10-20 thousand of them. To me this sounds more like a pixel. If you had a dead pixel on your monitor you probably wouldn't notice, and if you saw 10-20 thousand you may be able to make out what the picture was of. If we compare sample rate to pixel rate, then the shoe is definitely on the other foot in terms of numbers. 48,000 a second doesn't really compare to 414,720 (SD PAL Footage), 5,184,000 (1080p25 Footage) or a staggering 209,715,200 pixels per second (4k 25p Footage). So take that audio guys.

I'll talk a bit about bit depth in audio and video in my next post and tackle the mind boggling arena that is floating point as well. Stay tuned, if you're a massive geek like I am.

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