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Topic starter
Multistandard TV receivers usually had to deal with two (or more) intercarrier frequencies (and sometimes with AM as well as FM sound).
Sometimes separate intercarrier strips were used, one for each of the frequencies. Each would have its own ceramic bandpass filter, and its own quadrature coil.
Another approach was described by Philips, Germany, in a 1991 IEEE paper (1).
Here a wideband intercarrier strip was used, with a wideband PLL-type FM demodulator. The latter could, without any adjustment, accommodate any carrier frequency between 4 and 8 MHz. This range included the standard intercarrier numbers of 4.5, 5.5, 6.0 and 6.5 MHz. Nevertheless, system-specific bandpass filters were still required.
This was part of a single multistandard VIF-SIF IC, shown here in block schematic form:
The FM intercarrier PLL section is shown in more detail here:
Except in respect of FM demodulation, this IC followed conventional practice, with the intercarrier derived in the vision demodulator. Thus, it did not use QSS or other means to obtain improved sound quality.
Also, the IC itself catered for only one sound channel, although a second channel, as required for Zweiton, could be tapped off after the vision demodulator and processed in a separate IC. AM sound (for system L) also required external processing.
Nonetheless, the use of FPLL vision demodulation was claimed to be advantageous in respect of intercarrier buzz performance, as shown in this chart:
It was claimed to be about 10 dB better on intercarrier buzz than a would be obtained with passive video demodulation. Presumably the latter meant the quasi-synchronous type. But by 1991, PLL had probably become the norm, at least for higher quality systems.
Note that the Philips IC at interest was not identified by its marketing designation in the paper.
(1) An Advanced 5V VIF-SIF PLL tor Signal Detection in TV Sets and VTRs
Joachim Brilka, Joachim Keibel and Wolfgang Weltersbach’ Philips Semiconductors, Hamburg, Germany
IEEE 1991 August
Cheers,
Steve
Posted : 09/11/2024 5:35 am
Topic starter
An alternative approach to the multiple intercarrier system was adopted by Siemens in an IC also described in a 1991 IEEE paper (1).
Here the VIF-SIF IC was of the quasi-parallel type on the sound side, with only the sound carrier(s), without vision carrier, fed into the sound IF amplifier. Thus, it appears to have been aimed at a performance level above that of the Philips IC discussed in the previous post.
As in the Philips case, the VIF used an FPLL demodulator. In this case though, the VCO ran at twice VIF, rather than half VIF. The output of the VIF FPLL was used to generate the sound intercarrier(s).
Unlike the Philips case, AM sound for system L was demodulated on the IC, and there was provision for two FM intercarrier channels, to allow for Zweiton stereo.
The novelty in this case was the use of a second conversion for the SIF, as shown in this diagram:
This second conversions was done using an externally tuned, onboard VCO. The latter was tunable over the range 10 to 12 MHz. This was said to allow conversion of all intercarrier frequencies, namely 4.5, 5.5, 6.0 and 6.5 MHz, to a single frequency. The single frequency was not specified, but logically it would be one of those four, with 5.5 MHz being typical. In that case, 4.5, 6.0 and 6.5 MHz would be converted to 5.5 MHz using respective VCO frequencies of 10.0, 11.5 and 12 MHz, with 5.5 MHz itself unconverted. But other combinations could be used, except that it was not possible to convert either way between 6.0 and 6.5 MHz. The VCO and mixer could be switched out for one standard intercarrier frequency. Second sound channels (for Zweiton) would retain their relationship to their respective primary channels. The IC had an “unconverted” intercarrier output for feeding say a NICAM decoder. Each intercarrier FM channel had a low distortion quadrature demodulator.
This table shows the frequency conversion possibilities:
On intercarrier buzz, the following was said:
“For the intercarrier buzz we achieved the best results using loop bandwidth of the external loop filter of about 15 kHz. Because of the use of inexpensive standard tuners it is necessary to set the bandwidth to 100 kHz to avoid video carrier phase modulations within the tuner such as oscillator pulling or phase noise of the varactor diodes. The above makes it necessary to achieve a compromise with respect to intercarrier signal-to-noise-ratio (SNR). The result is shown in figure 4 as a function of detuning.”
That situation I imagine stems from Nyquist slope effects, which move from negligible at 15 kHz loop bandwidth to noticeable at 100 kHz. That does suggest that where the best possible intercarrier results were required, some form of Nyquist slope cancellation was preferable.
Note that the Siemens IC at interest was not identified by its marketing designation in the paper.
(1) A World Standard Video and Sound IF IC
R. Heymann and H. Kriedt
Siemens AG, Germany
IEEE 1991 August
Posted : 09/11/2024 5:51 am
Topic starter
Yet another approach to the multiple intercarrier system was adopted by Thomson in an IC also described in a 1991 IEEE paper (1).
This was an SIF-only IC of the QSS type. The VIF was also fed into the IF amplifier, along with the SIF, using the typical QSS double-peak SAWF which did not apply the Nyquist slope to the vision carrier. The latter was used as a reference for the PLL, which in this case operated at half VIF. The regenerated vision carrier was then used in quadrature to generate the intercarrier(s) in the first mixer. AM sound for system L was demodulated directly.
As in the Siemens IC discussed in the previous post, the FM intercarrier(s) were subject to another frequency conversion. The main FM intercarrier – which would be 4.5, 5.5, 6.0 or 6.5 MHz - was translated to 500 kHz, using supradyne conversion, in the second mixer. The required local oscillator signal was generated in a PLL. Using the same local oscillator signal, the second (Zweiton) intercarrier channel, where present, was converted to 240 kHz. These two IFs were then processed on the IC using integrated filters and demodulators. This avoided the need for external bandpass filters.
The resultant demodulated intercarrier signal to noise ratio was quoted as better than 55 dB on a CCIR peak basis. That would most likely refer to the basis for measurement, using the CCIR 468 weighting curve and the quasi-peak reading meter. The associated vision conditions were not mentioned, but one would expect it to have been worst case or perhaps white level (for the negative modulation case).
Note that the Thomson IC at interest was not identified by its marketing designation in the paper.
(1) Advanced Multistandard TV-Sound IF Integrated Circuit
M.A. Rieger and R.K. Koblitz
Thomson Consumer Electronics R&D Laboratories
Cheers,
Steve
Posted : 09/11/2024 5:54 am
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