Audio Decoders

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[edit] 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

[edit] 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)

[edit] 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).

[edit] 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
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