Coded data generation or conversion – Digital code to digital code converters – Byte length changed
Patent
1992-03-11
1994-02-01
Hoff, Marc S.
Coded data generation or conversion
Digital code to digital code converters
Byte length changed
341 77, H03M 700
Patent
active
052835774
DESCRIPTION:
BRIEF SUMMARY
The invention relates to a system for converting a PCM input signal with uniform noise spectrum, comprising a series of digital words each of K-bits appearing with a predetermined repetition frequency, into an PCM output signal with a non-uniform noise spectrum comprising a series of digital words each of L-bits, L being small than K which system comprises at least a 1-bit coder embodied to provide a shaped noise profile and to convert the K-L least significant bits of each word from the input signal into a series of 1-bit words, and an accumulator for combining said 1-bit words with the L most significant bits resulting into the desired PCM output signal.
Such a system is described in the U.S. Pat. No. 4,006,475. This patent publication describes in fact a further development of the system as described in U.S. Pat. No. 4,467,316. In both prior art systems use is made of "oversampling" because the frequency with which the K-L least significant bits of each word from the input signal are converted, is several times higher than the repetition frequency with which the words appear in the PCM input signal. In the system described in U.S. Pat. No. 4,467,316 use is made of a first order filter in the coder, resulting into the fact that the output signal has a non-uniform noise spectrum. Especially a shift is created from the lower frequency part to the higher frequency part of the spectrum with the result that on balance after filtering the baseband (the band which contains the desired useful signal parts) at the output of the system by means of a suitable filter the signal-noise ratio in the baseband is improved. However, it has appeared that this improvement is at the expense of additional oscillations in the output signal.
A further development is described in the U.S. Pat. No. 4,593,271. The therein described system comprises besides the already mentioned 1-bit coder a second 1-bit coder which is used for coding the residual value words generated by the first 1-bit coder. The resulting series of 1-bit words is, after differentiation in a thereto suitable circuit, combined with the series of signals at the output of the first summing circuit and is thereafter supplied to a filter to filter out the desired baseband. Just as the first 1-bit coder in this configuration also the second 1-bit coder operates at a frequency which is a multiple of the repetition frequency the PCM input signal.
Measurements have shown that with such a cascade circuit a better result can be obtained then with the embodiment comprising only one 1-bit coder as described in the above-mentioned U.S. Pat. No. 4,467,316 without introducing lots of other disadvantages. According to measurements the cascade circuit provides a shaping of the quantization noise with a spectrum having a slope of +12 dB/octave.
All the prior art systems are ultimately destined to convert the incoming PCM signal into an analog output signal, especially an audiosignal. If the PCM input signal comprises words with a relative large number of bits then the conversion of such a signal would require an D/A-converter with a relatively large number of discrete levels. Such converters are hard to realize. By using the above described systems words can be used with a smaller number of bits (although appearing with a much larger repetition frequency than in the original PCM signal), which can be converted into an analog output signal by means of a relatively simple D/A-converter.
In all these prior art systems consistently use is made of oversampling to be able to provide information, stored in the binary levels which are not present in the output signal anymore, in another manner to the D/A-converter. The result thereof is that even with a relatively low repetition frequency in the PCM input signal very high switching frequencies may appear in the system itself.
In U.S. Pat. No. 4,467,316 the attention is drawn in column 2, lines 45-49 to a PCM audio system functioning with a 16 bit converter and a sampling frequency of 50 kHz. Because of the applied oversampling in the accu
REFERENCES:
patent: 4006475 (1977-02-01), Candy et al.
patent: 4467316 (1984-08-01), Musmann et al.
patent: 4593271 (1986-06-01), Candy
patent: 4994804 (1991-02-01), Sakaguchi
Hoff Marc S.
Technische Universiteit Delft
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