Re: [Discuss-gnuradio] using ADC of a TV-card with CX2388x chip or digital TV-reception chips/cards

[Martin Dvh]

 > I used Audacity to record those samples but are
> annoyed by the extra harmonics generated during no
> input. The input is from the onboard Philip FI1213
> compatible tuner composite audio ouput. You can see
> the 19 kHz pilot  stereo signal, the L-R signal and
> the RDS data. Abandoned the idea of trying to decode
> the stereo and RDS data since the SNR is just about
> 6-10 dB?? above noise floor. Not sure whether this is
> good enough or not.
I found out that if you disable the clock-PLL (which you only-need for 
PAL-video capturing, not for NTSC and not for audio)
You get a much lower noise floor and less distortion / harmonics.
The driver automaticly disables the pll if you do a norm-switch to NTSC (do a 
short NTSC video capture).
You can check by looking at dmesg output. It should contain a line with:
"PLL can sleep, using XTAL"

For those interested. The pll is put to sleep by setting PLL_XCI register to 
zero.
0x0F8 Integer Register (PLL-XCI)
[5:0] RW 000000 PLL_I PLL_I input (1) . Range 6 63. If set to 0x00, then the 
PLL sleeps.

> At higher sampling rate, I think I notice more
> harmonics/noise. I wondering now whether the audio ADC
> can achieve more than 200k sampling rate. Can't
> remember what the datasheet says but am thinking
> either it is like that or it might be noise due to
> poor PCB layout.
The datasheet says it can't do higher sample rates but in practice it does.
You can also enable an additional 6dB signal boost amplification (meant for 
microphones)

0x10C Audio Control Register (GPIO_DMA_CTL)
[27] RW 0 A_G2X Audio gain boost. 0 = Normal gain setting as specified in 
A_GAIN (0.5 V rms standard input).
                                  1 = Adds +6 dB input signal boost from 
pre-amp.



> I exchanged the new TV card with Pixelview PlayTV
> Ultra which has a CX23881 chip on it. If the CX23883
> does support raw video ADC, this will really be great.
> If it is pin compatible with CX23881, I  might even be
> tempted to replace the chip. Hopefully the video ADC
> samples much higher than the tuner IF which is about
> 30 something MHz if I remember correctly.

The datasheet  for cx23880 cx23881 cx23882 cx23883 says:

The CX23880 family is designed to be a higher quality, more flexible, and configurable successor to the previous generation Fusion 878A. The key 
differences between the Fusion 878A and the CX2388x family are summarized in Tabl e 1- 1.

The pin and software compatible nature of the CX2338x family enables a board vendor to build a wide variety of analog and digital TV capture 
products from a common code base.
The CX23880 and CX23882 are designed to enable high-functionality broadcast- centric PC cards that require high speed I/O capability. This 
capability is necessary to support simultaneous compressed and uncompressed digital video/audio data flows in conjunction with hardware MPEG 
II/MPEG IV encoders and decoders.
The CX23382 has the same feature set as the CX23880, with the exception of BTSC 
stereo with dbx audio companding and VIP Host Port.
The CX23881 and CX23883 are designed to enable entry-level analog TV and stereo broadcast PC cards. Additionally, TV channel demodulators can be 
connected to enable software-based decoding of MPEG transport streams. The CX23881 has the same feature set as the CX23882, with the exception 
of BTSC stereo with dbx audio companding.

Table 1-1. Feature Comparison of CX2388x and Fusion 878A
Conexant PCI Decoder             878A CX23881 83 82 80
Standard Features Fusion
4-input CVBS ADC                   X       X  X  X  X
Chroma ADC (S-Video)               X       X  X  X  X
NTSC/PAL/SECAM Video               X       X  X  X  X
Multiple YCrCb and RGB Formats     X       X  X  X  X
Notch and Comb Y/C Separation      X       X  X  X  X
Serial MPEG Input Port             X       X  X  X  X
I 2 S I nput Port                  X       X  X  X  X
24-bit General Purpose I/O         X       X  X  X  X
Number of DMA Channels             2       7 11  7 11
WHQL Certification                 X       X  X  X  X
10-bit Video ADCs                          X  X  X  X
Multi-Line 2D-Adaptive Comb Filter         X  X  X  X
High Speed Parallel MPEG Port              X  X  X  X
I 2 S Output Port                          X  X  X  X
Stereo Line Out DACs                       X  X  X  X
EIAJ/NICAM/A2/FM Stereo Decode             X  X  X  X
BTSC with dbx Stereo Decode                   X     X
ITU-R 656 Output Port                            X  X
ITU-R 656 Input Port                             X  X
Intel/Motorola Host Port (I/O)                   X  X
VIP 2.0 Host Master Port (I/O)                      X


Greetings,
Martin