Audio Decoders

TV Sound Standards

TV-System	Sound Carrier [MHz]	Sound Modulation	Color System	Country
B/G	5.5/5.7421875	FM-Stereo	PAL	Germany
B/G	5.5/5.85	FM-Mono/NICAM	PAL	Scandinavia,Spain
L	6.5/5.85	AM-Mono/NICAM	SECAM-L	France
I	6.0/6.552	FM-Mono/NICAM	PAL	UK
D/K	6.5/6.2578125 D/K1 6.5/6.7421875 D/K2 6.5/5.85 D/K/NICAM	FM-Stereo FM-Stereo FM-Mono/NICAM	SECAM-East	USSR Peoples Rep. of China Hungary
M M-Korea	4.5 4.5/4.724212	FM-Mono FM-Stereo	NTSC	USA Korea
Satellite	6.5 7.02/7.20	FM-Mono FM-Stereo	PAL	ASTRA

TV Sound Decoding

The TV sound needs to be decoded in order to receive an Audio Frequency (Sound_AF) signal.

There are various possibilities

• Sound_AF provided by the tuner
  Some tuners have mono out /stereo out /radio out

• Sound_IF provided by the tuner, this needs an extra decoder
  • discrete decoding module (e.g. NE41S fpr BTSC stereo)
  • simple audio decoder chip (e.g. tda9820)
  • DSP audio decoder chip (e.g. MSP3415)
    • all DSP audio chips provide Sound_AF
    • some can provide I2S signal
  • Audio decoding function built into the PCI bridge chip (e.g. cx25840)

Sound Routing

There are multiple sound sources on the TV card, as there is only one output, there must be a switch to select from these:

• Sound_AF from tuner (mono/stereo/radio)
• Sound_AF from decoder chip
• Line In

There can be these switches (aka mux or crossbars)

• HEF4052: this selects from 4 inputs
• some DSP audio decoder chips have multiple inputs
• some PCI bridge chips have multiple input

If wired propoerly by the IHV, the I2S can be read directly by the PCI bdrige chip (so you have one less D/A -> D/A conversion, this should result in less noise).

Sound Output

Sound can be output to:

• Line out
  • Line out connected to sound card
  • Line out connected to active speakers
• Speaker or headphone out (some cards had a headphone amplifier)
• A/D digitizer on PCI chips (e.g. btaudio), this can be directly transferred to memory or can be sent to the audio card for output
• also see I2S above