Motion video signal processing for recording or reproducing – Local trick play processing – With randomly accessible medium
Reexamination Certificate
1997-03-14
2001-09-04
Chevalier, Robert (Department: 2712)
Motion video signal processing for recording or reproducing
Local trick play processing
With randomly accessible medium
C386S349000
Reexamination Certificate
active
06285822
ABSTRACT:
BACKGROUND OR THE INVENTION
1. Field of the Invention
The invention relates to the field of recording and/or replaying standard video data using a video recorder.
2. Description of the Related Art
It is well known that it is not possible to reliably record and replay teletext data using a standard VHS video cassette recorder, principally because the bandwidth of the VHS channel is very limited. The luminance bandwidth is 2.9-3 MHz which can be reduced to 2 MHz on playback when the sharpness control is set to “soft”. As a comparison the bandwidth of a normal broadcast video channel is of the order of 5.5 MHz. The limited VHS bandwidth has proved acceptable with normal video signals, the effect being to soften or blur the picture to an extent which has proved to be acceptable to most viewers. When teletext was first specified, however, a high bit rate was chosen to make maximum use of the broadcast bandwidth. Unfortunately, this means that the high frequency part of the data signal, including the very important clock run-in signal, is obliterated by the recording and replay process. In addition most teletext decoders rely on the very close line timing tolerances achieved by broadcast transmissions. The decoders can be completely upset by synchronising jumps created just before the vertical blanking interval (VBI) by the recorder head switching. Further problems are caused by the variation in line frequency caused by stretched tapes which can cause a variation of ±3% in line frequency.
It has been proposed in EP-A-0 608 960 (PHB 33832), to U.S. Pat. No. 5,565,997, issued Oct. 15, 1996, that teletext data should be recorded on VHS tapes at a lower bit rate by decoding the teletext data and expanding the data packets by a factor of four. This data is then spread across five VBI lines, to allow for guard periods. On replay the data packets from the five lines are compressed to restore the data rate to that in the teletext specification to enable display of the data.
The above citations are hereby incorporated herein in whole by reference. Such an arrangement enables the teletext data to be recorded on and replayed from a VHS recorder but suffers from the disadvantage that it is impossible to record all the teletext pages that are transmitted as there are insufficient VBI lines to accommodate them. Consequently a selection of the pages to be recorded has to be made at the time of recording and as a result only those selected pages can be displayed when the tape is replayed.
A further proposal in GB-A-2132856 is that the teletext data be written into a buffer store at the standard teletext data rate and read out at half that rate for recording on tape. In this instance no selection of pages to be stored is made, but correct operation depends on there being at least twice as many VBI lines available as are used for teletext data transmission. This is not currently the case in the UK where more than half the available VBI lines contain teletext data or other data.
SUMMARY OF THE INVENTION
It is an object of the invention to enable the provision of a method of and apparatus for recording and/or replaying teletext data using a video recorder having a bandwidth of less than the bandwidth of a broadcast channel, as a result of which it is possible to record and/or replay all the teletext data transmitted.
The invention provides a method of recording teletext data on a record carrier using a recording apparatus having a bandwidth of less than the standard teletext data rate including the steps of
receiving teletext data at the standard data rate,
converting the teletext data to a multilevel code, at a data rate which falls within the bandwidth of the recording apparatus, and
recording the multilevel code on the record carrier.
By converting the teletext data to a multilevel code the data rate can be reduced. For example a four level (or quaternary) code allows two bits to be encoded for each data period and consequently allows the data rate to be halved. By this means the teletext data transmitted in one VBI line can be recorded to a single VBI line in the recorder without requiring broadcast bandwidth. Consequently it is then possible to record all the teletext data broadcast as part of a programme which is being recorded. This means that it is not necessary at the time the recording is being made to decide which teletext data to record and that this choice can be made from the total transmitted data at the time the programme is being replayed.
The multilevel code may include a signal having more than two amplitude levels. In this case the multilevel code is coded in terms of signal amplitude. An alternative would be to provide a multi phase signal, a four phase signal giving the same possibility of encoding two bits in each data period. Of course, a combination of phase and amplitude could be used to form the multilevel code.
The multilevel code may have eight levels and a data rate one third of that of the standard teletext signal. As is readily apparent to the skilled person an eight level code will enable three bits to be encoded in each data period and will consequently allow the data rate to be reduced to one third of the data rate of a binary signal.
The invention further provides a method of replaying teletext data from a record carrier using replay apparatus having a bandwidth of less than the standard teletext data rate, the teletext data being recorded on the record carrier by means of a multilevel code at a data rate which is lower than the standard teletext data rate, the method including the steps of
reading the multilevel code from the record carrier,
converting the multilevel code to standard teletext data, and
applying the teletext data to a teletext encoder.
The data recorded on the record carrier may be by the method according to the invention when the recording apparatus is used to record a broadcast programme (including cable television), or may be inserted on the record carrier from a pre-recorded tape or disc.
The invention still further provides apparatus for recording teletext data on a record carrier, the recording apparatus having a bandwidth of less than the standard teletext data rate, the apparatus including apparatus for receiving a video signal including teletext data, apparatus for detecting received valid teletext data, apparatus for encoding the received teletext data into a multilevel code at a data rate which is less than the standard teletext data rate, and apparatus for recording the multilevel code on the record carrier.
The apparatus for encoding teletext data may include apparatus for applying the received teletext data to an encoder in n-bit packets, where n is an integer greater than one, apparatus for converting each n-bit packet into a multilevel code having at least one level for each n-bit combination, and apparatus for feeding the multibit code to the record head of the recoder at a data rate of 1
times the standard teletext data rate.
The packets may be of two or three bits, which correspond to four or eight level codes respectively. The four level code allows the data rate to be halved whereas the eight level code will allow it to be reduced to one third of the original rate. The more levels allowed the greater the reduction in data rate but the greater the difficulty in distinguishing between them.
A buffer RAM may be connected between the apparatus for detecting teletext data and the encoder.
This enables timing to be simplified. It will be apparent that some time is needed to detect that teletext signals are being received on any given VBI line. There fore, to encode and reinsert the data on the same VBI line would require very fast decisions and encoding. By providing a buffer store the data can be reinserted either on a succeeding line of the VBI, if available, or on a line in the VBI of the next field or frame.
The buffer RAM may receive only teletext data packets, the encoder including apparatus for generating the clock run in and framing code.
This enables the size of the buffer RAM to be minimised, which may be particular
Belk Michael E.
Chevalier Robert
U.S. Philips Corporation
LandOfFree
Recording and/or replaying teletext signals does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Recording and/or replaying teletext signals, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Recording and/or replaying teletext signals will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2471986