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Digital Video Encoder IC's.




   Digital Video Encoders   (aka DVE)
   ----------------------

  All digital satellite receivers require an analogue output
  to send to the viewing device (normally a TV or monitor).
  The (digital) satellite receiver's back end,after all
  MPEG demultiplexing & decoding, outputs DIGITAL video &
  audio streams.
  
  These need converting to standard PAL,NTSC or SECAM analogue
  video signals (or to RGB).This is done in a DVE.
  The audio data is converted using audio Digital to Analogue
  converters similar to those used in CD players.
  The video encoder is more complex due to the many different
  analogue video standards that exist as well as the extra 
  add on features needed like on screen menus & teletext.
  A DVE is basically a number of DAC's (Digital to Analogue
  Converters) along with appropriate digital filters,timing
  generators etc.
  The digital video input is in a format known as CCIR601,
  the commonly accepted standard for broadcasted video. 
  If only composite video was required,just 1 DAC would be 
  needed.However,today's consumer also wants RGB & S-VHS
  outputs & may want some these simultaneously.As a result 
  most DVE's have 3 (& some have more) DAC's on the chip.The DAC's
  have to operate at fairly high speed - typically 140MHz.
  Cheaper DVE's use 8 bit DAC's,better ones use 10 bits -i.e.
  a resolution of 1 part in 1024.
  The output of the encoder has to be able to produce video to
  many variants of standards so that it can be used in as many
  regions or countries as possible.This means that the on-chip
  filters & oscillators have to be very flexible.
  Inputs have to be provided for field & line syncs,blanking &
  clock which are fed to a complex timing generator.
  They also have to be able to support Closed Captioning &
  teletext.
  As an example,here is a table showing simultaneously available
  video output options for the Analog Devices ADV7176 (10 bit) DVE :-

  MATRIX of Simultaneous Video Output options
  -------------------------------------------

          Option1    Option2     Option3   Option4  

  DACa    CVBS        Y            CVBS       -     
  DACb    CVBS(rf)    CVBS(rf)     B          U        
  DACc    C           C            R          V
  DACd    Y           CVBS(rf)     G          Y	
 
  CVBS is Composite Video Blanking & Sync.
  CVBS(rf) is the same but intended for feeding to an RF modulator.
  Y/C is separate Y (luminance) & C (Chrominance) as used in S-VHS
  VCR's.
  YUV uses colour difference signals + separate luminance -normally 
  used for studio/professional applications ("component video").
  The same IC has to produce PAL video with the 4 different subcarrier
  frequencies (PAL -BDGHIN,PAL-Nc,PAL-M,PAL-M-60Hz are all different;
  it also produces a similar number of NTSC modes.
  The chip uses a serial digital input (I2C) to set up its modes via
  a large number of registers.
  If the incoming signal has teletext information ,this must be passed
  through to the TV (in the vertical blanking interval).
  The DVB/MPEG2 specification also allows for a separate teletext
  stream.If text is transmitted in this way,the encoder must have
  "teletext insertion" capabilities.
  Many DVE's also have a built in colour bar generator (for test
  purposes).

  Macrovision
  -----------
  Some DVE's have built in Macrovision -a form of Pay Per View copy
  Protection (PPV-CP).This system allows viewing on a TV but ruins 
  picture quality if recorded on a VCR.Pulses are sent in/around the VBI
  (the vertical blanking interval - as also used for teletext data).
  This confuses the agc (automatic gain control) in the VCR by turning 
  down its gain ; results vary between VCR's but expect dim pictures
  noise,loss of colour,saturated colour or loss of line/frame lock.
  A second Macrovision component modifies the colour signal to cause
  colour striping on replay.
  There is a similar "anticopy" system for DVD players.The manufacturers
  of the receivers have to get a licence from Macrovision Corporation
  to incorporate these features;they then use the "Macrovision-enabled"
  version of the DVE chip in receivers where this is required.Most
  current European digital boxes do NOT incorporate this -but some
  future ones will certainly do so. If the Operator/provider wants
  to be able to screen films over digital satellite before they're
  released in the video shops,the film rights-owners would insist
  that the Operator's subscribers have Macrovision!!
  This already happens on some cable boxes in the USA.

  If you want to get a full datasheet for a DVE,try the Websites
  (or databooks) of Philips,Analog Devices,SGS-Thomson or Brooktree.
  Some part numbers are as follows :
  Brooktree : BT851,BT852. BT856,BT857
  Philips   : SAA7182,SAA7183
  Analog Devices : ADV7175,ADV7176.
  SGS-Thomson STV0117,STV0118,STV0119.
   
  If you look at any of these datasheets,you'll be amazed how complex
  these chips are -yet their price in volume to the Receiver
  manufacturers is only around 4 to 5 US dollars.
  Future DVE's may also contain the audio DAC's or,alternatively,the
  function may be integrated into the (already complex) MPEG2 decoder
  chips.

 Chris Muriel, July 3rd,1997.
 (chris.muriel@analog.com)
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