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8 November 2011 Analog v. Digital Technology
To: coldwarcomms[at]yahoogroups.com On Tue, Nov 08, 2011 at 07:13:25AM -0000, widebandit wrote:
> Even from the early days of TD2, video and audio were delivered
over Interesting bit of history, but by the 70s it became apparent that it would be much nicer to deliver video and audio over the same path and avoid all sorts of issues in the process and this led to the FM subcarrier over video scheme that was used in the Bell System from then onward and which also quickly became the standard method of transmitting video and audio together on analog satellite circuits. There has been almost zero satellite transmission of separate video and audio signals for television since this subcarrier audio transmission system was developed, over the years since the inception of domestic satellite transmission in the mid 70s almost every scheme used on these satellites involved multiplexing both audio and video on one signal and sending them via a common path. Various schemes have been used, encrypted digital sound in the sync time was standard for analog scrambled videocipher cable signals for many years, and various pulse position modulation schemes were used in the very early days - also in the sync... but FM subcarriers remained the most standard analog technique for transmitting sound. When digital satellite transmission came along and replaced analog the whole MPEG transport stream scheme was invented to allow all kinds of signals (closed captions, subtitles, time codes, data transmissions, control information, picture format and letterbox cropping information, program guide data, and multiple channels of audio in different languages) to ride along in one signal along with the video. I am unaware of many cases where anything else has been done to transmit sound though modern technology certainly allows multiple channels of audio including some in different encoding formats to be present at once. Within broadcast plant signals carrying audio and video together are also common (HDSDI with embedded audio in particular these days) though of course there still are many places where audio goes through separate processing boxes and paths and switchers and mixers than the video does. These days in addition to satellite or microwave transmission of digital video transport streams more and more video is being sent encapsulated in IP streams over either dedicated fiber paths or even regular Internet broadband connections. Mostly these video over IP schemes involve encapsulating the packets of a normal MPEG transport stream in IP datagrams... and sending them over a more or less normal IP network path over fiber. More and more backhauls of sports events such as NFL games are sent back to the studio this way these days. And almost universally the audio is multiplexed into the same transport stream with all the other streams including the compressed video - usually the same gear is used to compress and multiplex (and decode at the other end) the video and audio streams into a transport stream as is used over satellite... and sometimes a satellite circuit is used as backup for the fiber carrying essentially the same transport stream. --
Dave Emery N1PRE/AE, die[at]dieconsulting.com DIE Consulting, Weston, Mass
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To: coldwarcomms[at]yahoogroups.com While most of this fascinating discussion has been on the TV, telco, & satellite network portion of the signals, be aware, up until the transition to digital TV, your local broadcaster used an FM transmitter for the sound and an AM transmitter for the video on close, but separate frequencies. That electronic separation continued all the way to your screen and speakers. Even your own head uses two different types of "receivers" to get the full "signal". While this has nothing to do with the original question about remote delays, it's worth mentioning in a historical sense how the sound and video, being drastically different types of signals with matching processing equipment (including long haul), maintained that logical separation of sound and video for a very long time. David just added an excellent description of more recent developments in the system that allowed combining signals via FM subcarrier into "one" signal, and the further progression to digital. Not much to add to that other than a footnote that as long as there is light (fiber) or RF (satellite, broadcast, cable, DSL, wi-fi, or even cell phone) in the transmission path, it's an analog medium transporting digital information. That's just the physics of it, regardless of how many sources misrepresent it as being anything else. Mike
Cryptome: Analog media -- computers, chips, human interface devices, encryption passphrases, monitors, peripherals, wiring, cables, fiber, routers, switches, transceiver antenna (earth and satellite) -- are the most frequently exploited weaknesses of comsec, exceeded only by analog human bodies multiplexing EM signals into digital programs vainly struggling to prevent analog bit rot, mortality.
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