Over-sampling A/D converting circuit
View Patent ↗To provide an A/D converting circuit capable of improving an S/N ratio regardless of a color television system. An over-sampling A/D converting circuit according to an embodiment of the present invention includes: an A/D converter for over-sampling an analog video signal; a digital low-pass filter allowing passage of a signal component in a predetermined passing band out of the over-sampled signal; a down-sampling circuit for down-sampling the passed signal; and a color television system determining circuit for switching the passing band.
1. An over-sampling A/D converting circuit, comprising:
an input receiving an analog video signal; and
a filter unit limiting a frequency band of the analog video signal in accordance with a sampling frequency that varies depending on a color television system of the analog video signal,
wherein the filter unit includes a plurality of frequency band limiting filters each having a predetermined passing band based on the sampling frequency in accordance with the color television system, the frequency band limiting filter suitable for the sampling frequency being selected from among the plurality of frequency band limiting filters.
2. The over-sampling A/D converting circuit according to claim 1 , wherein unselected frequency band limiting filters out of the plurality of frequency band limiting filters stop operating.
3. An over-sampling A/D converting circuit, comprising:
a first sampling unit executing A/D conversion on an input analog video signal with a first sampling frequency;
a frequency band limiting filter unit allowing passage of a signal component in a predetermined passing band out of a signal passing through the first sampling unit;
a second sampling unit sampling the passed signal with a second sampling frequency; and
a switching unit switching the passing band of the frequency band limiting filter unit,
wherein the switching unit determines a color television system of the analog video signal based on the signal sampled with the second sampling frequency to switch the passing band of the frequency band limiting filter unit in accordance with the determined color television system.
4. The over-sampling A/D converting circuit according to claim 3 , wherein the switching unit switches the first sampling frequency and the second sampling frequency.
5. The over-sampling A/D converting circuit according to claim 3 , wherein the switching unit switches the passing band of the frequency band limiting filter unit in accordance with the second sampling frequency.
6. The over-sampling A/D converting circuit according to claim 3 , wherein the first sampling frequency is higher than the second sampling frequency.
7. The over-sampling A/D converting circuit according to claim 3 , wherein the frequency band limiting filter unit is a low-pass filter that allows passage of a signal component in a frequency range not higher than a cut-off frequency.
8. The over-sampling A/D converting circuit according to claim 7 , wherein the cut-off frequency is a Nyquist frequency of the second sampling frequency.
9. The over-sampling A/D converting circuit according to claim 3 , wherein the frequency band limiting filter unit includes a selection unit selecting a filter factor in accordance with the switching of the switching unit.
10. The over-sampling AID converting circuit according to claim 3 , wherein the frequency band limiting filter unit includes: a plurality of filters having different passing bands; and a selection unit selecting one of the plurality of filters in accordance with the switching of the switching unit.
11. The over-sampling A/D converting circuit according to claim 10 , wherein the selection unit stops an operation of unselected filters out of the plurality of filters.
12. The over-sampling A/D converting circuit according to claim 3 , further comprising:
a first clock generating circuit generating a clock of a first clock frequency in accordance with the switching of the switching unit to apply the clock of the first clock frequency to the first sampling unit and the frequency band limiting filter unit; and
a second clock generating circuit applying a clock of a second clock frequency obtained by dividing the clock of the first clock frequency to the second sampling unit.
13. The over-sampling A/D converting circuit according to claim 12 , wherein the first clock frequency is the first sampling frequency, and the second clock frequency is the second sampling frequency.
14. A method of sampling an analog video signal, comprising:
receiving the analog video signal; and
limiting a frequency band of the analog video signal in accordance with a sampling frequency that varies depending on a color television system of the analog video signal,
wherein the limiting step comprises:
passing the analog video signal through a filter unit that includes a plurality of frequency band limiting filters each having a predetermined passing band based on the sampling frequency in accordance with the color television system; and
selecting a frequency band limiting filter suitable for the sampling frequency from among the plurality of frequency band limiting filters.