28 April, 2008

It’s still an Analog world

“Television is great for engineers, you get to deal with a frequency range of DC to beyond daylight, power levels from Pico Watts to Mega Watts, computers and software that run the gamut of user indifferent to downright user hostile, with enough analog left to require some precision”

Another two month lacuna in blogging. I must admit continual wonder at those who are such prolific posters, but then I realize real life and work demands are such that this is (for me) a bit of a luxury – while I spent an inordinate amount of time parked in front of a monitor and keyboard I’m often not connected (for a variety of reasons) to the outside world.

Recently completed the annual pilgrimage to that most improbable of places for the National Association of Broadcasters convention to Las Vegas. This year corporate had rooms at both the Marriott directly across the street from the convention center and the Wynn (having been informed that we were no longer welcome at the previous accommodations – still trying to get the back story on that one!) The Marriott was my first choice but setup crew had priority and due to various reasons I was not on setup or strike this year so ended up at the Wynn. Yes, it’s a very nice place, large rooms elegantly appointed, fancy eateries (didn’t try any except room service the first night - $60 for pasta with 4 prawns and a glass of white wine) and no coffee in room for less than $25 or stand in line – the room did have a decent flat panel TV that actually got a few HD programs (including PBS – although did not make it back in time to see Nova).

For the last decade or more the “Broadcaster” portion of NAB has become less and less a relevant portion of the show (and in light of the February ’09 cutoff date for Analog transmission could well become a historical oddity) not that the convention has become irrelevant (although Apple/Avid was not there) it is still the premier venue for anything to do with recording, creating & distributing moving pictures and sound – some portion of which may actually be broadcasted over the air – which brings me to the subject of this entry.

Until the mid ‘70’s with the first introduction of the Digital Time Base Corrector, every image, every sound was recoded, edited, played back and broadcast with analog technology. Even well in to the ‘90’s almost all recording and post production was done with analog decks, admittedly lots of digital devices and circuits became involved in the process but everything was still passing through analog I/O (Input/Output).

The first affordable digital tape decks did a rather simple 8bit quantization of the entire composite analog waveform providing only 256 levels of gray between black and white (actually something less that that as “00” and “FF” and the several steps ‘below black’ were not used) resulting in a digital approximation of the analog waveform. This digital approximation has several virtues, it can be replicated without further degradation an infinite number of times, stored on a variety of media and faithfully reproduced on demand and will always be the same, it is however only an approximation of the original analog waveform. In a pure digital signal path this would not present any problems but when passing back through that analog I/O and back to another digital approximation the opportunity for all sorts of distortions and errors arises requiring constant attention to that pesky analog portion of the signal.

In this 21st century, analog decks are still around although they are seldom (if ever) used in the more progressive facilities where once the image leaves the imaging device at the camera it stays digital throughout the entire production, post production and distribution stages only reverting to analog at the display, either on the input to the display (VGA or Component inputs) or when viewed with the ‘ol mk1 eyeball. This is not to say that the string of digits coming off of the Analog to Digital converters on the first stage of the camera electronics is unchanged in the process but digital compression is another series of rants I’ll not touch upon at this time.

In the (good old days? Not really, Tubes are interesting devices but B+ is a shocker!) Analog world sound and pictures were a continuous undulating waveform starting at the microphone or off the scanning beam of one of the three plumbicon tubes (OK, I’ll admit I know what an Image Orthocon is and that some cameras had 4 tubes and the scanned area had a several minute duration wobble to avoid burn in but that is ‘ancient’ technology) and at almost any point in the signal path you could look with an oscilloscope and see the current representation of the original analog waveform in all its nuanced glory (well, within the limits of frequency response and other analog woes) in the world of audio one could compare the waveform of the microphone and the waveform of the amplified signal to the speakers and match the two to several orders of magnitude – color television is a bit more complicated in that the method of encoding the color information on a composite signal requires determining the phase offset from a precise reference frequency, however the black and white signal is still recognizable once you know what you’re looking at.

But the ultimate fact remains – its still an analog world, the camera and microphone are analog devices, your hearing and vision are analog processes and if you really want to mangle an analog signal, do it digitally!

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