Video data compression apparatus for recording and reproducing compressed video data at their various compressed data rates

Apparatus for recording and reproducing video data is described that utilizes data compression to increase storage capacity. Real time input video data is decorrelated, quantized and entropy encoded. User control of a compression control parameter, such as quantization step width, is provided so that the user can adjust the loss of fidelity through compression to a desired level. The compressed data is output to a storage RAM 38 at an unregulated varying data rate. The system allows real time storage of video data in a manner in which the loss of fidelity is held constant and the amount of storage capacity required for particular sections of input video data varies with the information content of that input video data.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to video data compression. More particular, this 
invention relates to the real time recording of video data as compressed 
video data. 
2. Description of the Prior Art 
Data compression is utilised in the still image field. The proposed JPEG 
standard provides a non real time image data compression technique for use 
in situations such as the compression of still images by and for storage 
within general purpose computers. The JPEG technique utilises quantization 
as One step in the compression process. The quantization step width 
applied may be changed to change the degree of compression achieved. Using 
a Small quantization step width will increase the length of time needed to 
encode the data (limited rate data channels within a computer) and the 
amount of storage space needed to store the compressed image. Conversely, 
using a large quantization step width will decrease the length of time 
needed to encode the data and the amount of storage space needed to store 
the compressed image. The asynchronous, non real time nature of the JPEG 
approach means that such variation is not an issue of importance. 
It is also known to utilise real time compression systems in conjunction 
with storage applications in order to increase the storage capacity. A 
requirement in such systems is the achieving of a constant output bit rate 
compatible with the storage medium used. This approach results in a 
variable picture quality depending upon the image information content. 
Furthermore, since these systems have to achieve an acceptable picture 
quality with the most difficult of images, they produce an image quality 
in excess of that genuinely required for the majority of pictures with a 
consequential reduction in storage capacity. 
The constant data rate approach is imposed on convention compression 
systems due to the nature of the storage media that are generally used. In 
particular, storage media such as magnetic tapes, hard-discs and magneto 
optic discs have mechanical inertia such that they are only able to deal 
with a constant data rate when operating close to their peak performance. 
Thus, for images with a small amount of information content, the image 
quality exceeds the specification (and is likely to be lossless) and for 
images with a large amount of information, the image quality can be below 
specification. 
SUMMARY OF THE INVENTION 
Viewed from one aspect this invention provides apparatus for real time 
recording constant rate uncompressed video data, said apparatus 
comprising: 
(i) means for compressing said uncompressed video data to form compressed 
video data at a varying compressed data rate, said varying compressed data 
rate being unregulated and dependent upon a compression control parameter, 
which controls loss of fidelity through compression, and suitability of 
said uncompressed video data for compression; 
(ii) means for user adjustment of said compression control parameter; and 
(iii) means for storing said compressed video data at said varying 
compressed data rate; 
whereby, in use, a user may adjust said compression control parameter to 
balance loss of fidelity against maximum recording capacity for particular 
uncompressed video data. 
The invention moves against the trend in the field by recognising that 
significant advantages can be achieved if the compressed data rate is 
allowed to vary, i.e. the compressed data rate is unregulated and will 
vary as the input video data changes from images of high information 
content to images of low information content and vice versa. A requirement 
for this new approach is that the storage medium used be capable of 
accepting such variable rate compressed data. 
In operation, the user is able to adjust the compression control parameter 
to control the loss of fidelity through the compression system, i.e. in a 
post-production system where absolute image quality is vital, then very 
little loss of fidelity will be tolerated and the compressed data rate 
will be high, whereas in a situation, such as television sports 
broadcasting in which an instant replay is required, absolute image 
fidelity is not so important and this can be traded against an increase in 
storage capacity due to a lower compressed data rate. 
The principle of the invention can be thought of as allowing the user to 
select the amount of loss of fidelity they are prepared to accept during 
the compression process, this being held constant and the degree of 
compression achieved varying in dependence upon whether high information 
content or low information content images are being compressed. In 
contrast to this new approach, the previous approach was to maintain a 
constant degree of compression irrespective of the information content of 
the image concerned with a consequential variation in image quality that 
is often highly noticeable and visually disturbing. 
As mentioned above, the means for storing the compressed video data must be 
able to handle the varying compressed data rate. One possibility would be 
to use a storage medium operating significantly below its maximum data 
rate such that it is able to cope with the synchronisation and other 
problems associated with varying data rate. In this case, the storage 
medium could be chosen from amongst the conventional media such as magneto 
optic discs, hard-discs and magnetic tapes. As the maximum data rates that 
can be handled by such technology steadily increase, their use in this 
circumstance becomes increasingly easy. This is particularly true of 
magnetic disc storage, that has the advantages of ready availability and 
steadily increasing performance. In addition, it will be recognised that 
as the data rates associated with image data are typically very high, a 
storage medium inherently able to deal with varying data rates without 
difficulty will be a significant advantage. Accordingly, random access 
memory is particularly suitable for use as the means for storing in the 
present invention. 
In the case of the use of random access memory, it is convenient to control 
the storage by using a write controller for generating an periodic write 
control signal for triggering storage of a byte of compressed video data 
in said random access memory when said byte is output by said means for 
compressing and for incrementing a write address of said random access 
memory along a predetermined sequence. 
It will be appreciated that the compression control parameter could take a 
number of forms. For example, an image could be compressed in manner in 
which the high spatial frequency information was ignored so as to achieve 
a higher degree of compression. In this case, the amount of high spatial 
frequency information removed can be adjusted using a control parameter 
acting upon an appropriate filter. 
However, in particularly preferred embodiments of the invention it is 
advantageous that said means for compressing comprises a quantizer and 
said compression control parameter is quantization step width. 
The use of a quantizer with a variable quantization step width allows 
almost direct control over the loss of fidelity. 
Preferred compression systems also utilise a frequency separator for 
transforming the video data into the spatial frequency domain and an 
entropy encoder, such as a runlength and Huffman coder. 
A complementary aspect of the apparatus for recording the compressed video 
data is that it should be provided with means for reproducing the video 
data. Accordingly, preferred embodiments have means for real time 
reproducing said uncompressed video data at said constant uncompressed 
data rate from said compressed video data, said means for real time 
reproducing comprising: 
(i) means for reading said compressed data from said means for storing at 
said varying compressed data rate; and 
(ii) means for decompressing said compressed video data at said varying 
compressed data rate to form said uncompressed video data at said constant 
uncompressed data rate, said means for decompressing being controlled by 
said compression control parameter stored in association with said 
compressed data. 
The reproducing arm of the device advantageously includes a read controller 
for generating an periodic read control signal for triggering reading of a 
byte of compressed video data from said random access memory when said 
means for decompressing is ready for a next a byte of compressed video 
data and for incrementing a read address of said random access memory 
along said predetermined sequence. This corresponds to the write 
controller on the recording side. 
The reproducing arm also will include a complementary dequantizer, 
frequency combiner and entropy decoder. 
Viewed from another aspect this invention provides a method of real time 
recording constant rate uncompressed video data, said method comprising 
the steps of: 
(i) compressing said uncompressed video data to form compressed video data 
at a varying compressed data rate, said varying compressed data rate being 
unregulated and dependent upon a compression control parameter which 
controls loss of fidelity through compression, and suitability of said 
uncompressed video data for compression; 
(ii) adjusting said compression control parameter in response to user 
inputs; and 
(iii) storing said compressed video data at said varying compressed data 
rate; 
whereby, in use, a user may adjust said compression control parameter to 
balance loss of fidelity against maximum recording capacity for particular 
uncompressed video data.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 illustrates video data recording and reproducing apparatus utilising 
data compression. Video data is input to a compression apparatus 2 where 
it is first received in a decorrelator The decorrelator 4 applies a 
spatial frequency separation technique, such as wavelet coding, sub band 
coding or DCT coding. The output of the decorrelator 4 passes to a 
quantizer 6. 
The quantizer 6 applies quantization according to a given quantization step 
width to the decorrelated data in order to reduce the information content 
thereof. The quantization step width may vary between differing frequency 
components of the decorrelated data in accordance with their corresponding 
importance to the ultimate perceived image quality. On an overall level, 
the mean quantization step width applied is adjusted by a rate controller 
8. Applying more severe quantization will reduce the information content 
of the data and allow a higher degree of compression compared with 
applying a less severe quantization. The output from the quantizer 6 
passes to an entropy encoder 10. 
The entropy encoder 10 performs runlength coding and then Huffman coding in 
accordance with known techniques. An inherent characteristic of such 
encoding techniques is that they result in an output data rate which is 
dependent, among other things, upon the suitability of the image for 
compression. The output from the entropy encoder 10 is the compressed data 
stream. This compressed data stream is regulated to have a constant 
compressed data rate by a feedback action imposed by a buffer 12 and the 
rate controller 8. 
The buffer 12 receives the compressed data from the entropy encoder 10 
before passing it to a random access memory 14 (the buffer effectively 
smooths the data rate). The buffer 12 generates a control signal serving 
to perform negative feedback that is applied to the rate controller 8 such 
that if the buffer 12 becomes too full, then the quantization step width 
is increased and if the buffer 12 becomes too empty then the quantization 
step width is decreased. The data output from the buffer 12 to the RAM 14 
is is at a constant regulated compressed data rate. 
The data stored within the RAM 14 may subsequently be read out and 
decompressed by a decompression system 16. The compressed data is read out 
from the RAM 14 at a constant rate and passed to an entropy decoder 18 
where inverse Huffman coding and inverse runlength coding are applied. The 
constant rate data From the RAM 14 passes to the entropy decoder 18 via a 
buffer 20. The amount of free space within the buffer 20 is passed to a 
rate controller 22 which uses this to determine a quantization step width 
to be applied by a dequantizer 24 that receives the output of the entropy 
decoder 18. The symmetry of the system is such that the step width applied 
by the dequantizer 24 will follow that originally applied by the quantizer 
6. 
The output from the dequantizer 24 passes to a correlator 26 where the 
delta is transformed from the spatial frequency domain to the spatial 
domain and output as constant rate video data. 
FIG. 2 illustrates a video data recording and reproducing apparatus in 
accordance with one embodiment of the invention. Constant rate video data 
is input to a compressor 28. The constant rate video data first passes to 
a decorrelator 30 where it is transformed into the spatial frequency 
domain under the action of one of the known transformation techniques such 
as wavelet coding, sub band coding or DCT coding. 
The output from the decorrelator 30 passes to a quantizer 32 where it is 
subject to quantization by a quantization step width that is manually 
controlled via a user input device (not illustrated). This quantization 
step width constitutes a compression control parameter that controls the 
amount of fidelity lost during compression. The other parts of the 
compression system are lossless, whereas quantization necessarily involves 
a loss of information from the signal. The courser the quantization step 
width, the greater the loss of information, but the greater the degree of 
compression that can be achieved. 
The output from the quantizer 32 passes to an entropy encoder 34 where it 
is subject to runlength and Huffman coding. As each byte of data (a byte 
may represent whole and partial Huffman codes depending upon where the 
boundaries between the Huffman codes happen to fall) is assembled by the 
entropy encoder 34, a write enable signal is passed to a write address 
generator 36. The write address generator 36 under control of the write 
enable signal and a clock signal generate an enable signal for writing to 
a RAM 38 at an address that follows a predetermined sequence of addresses. 
The data present at the output of the entropy encoder 34 is then written 
into the RAM 38. 
As the degree of quantization is set under manual user control, the 
quantization and the loss of fidelity are constant until the quantization 
step width is manually changed. This has the result that there is less 
perceivable difference in quality between images of high and low 
information content. Another consequence of this approach is that the data 
rate of the compressed data from the entropy encoder 34 varies in 
dependence upon whether the input video data has a high information 
content or a low information content. 
In the case of low information content input video data, relatively few 
Huffman codes will be needed to represent it and the compressed data rate 
will fall thereby requiring less space within the RAM 38 to store a given 
time period of video data. Conversely, as the information content of the 
input video data increases, the compressed data rate also increases and 
more space within the RAM 38 is required. 
On the decompression side of the apparatus, the data is read from the RAM 
38 under control of a read address generator 40. The read address 
generator 40 receives a read enable signal from an entropy decoder 42 
indicative of the entropy decoder being ready to receive the next byte of 
data for the coding. This read enable signal is then combined with a clock 
to produce an enable signal for the RAM 38 triggering reading of a byte of 
data from the RAM 38 at the address specified by the address generator 40 
and incrementing of the read address along the same predetermined sequence 
as that used on the compression side. This byte of data is passed to the 
entropy decoder 42. 
The entropy decoder 42 applies inverse Huffman coding and inverse runlength 
coding to the data and passes its output to a dequantizer 44. The 
dequantizer 44 receives the quantization step width to be used in the 
dequantization directly from within the data read from the RAM 38. This 
dequantization step width is that which was manually specified by the user 
during the compression. The output from the dequantizer 44 passes to a 
correlator 46. 
The correlator 46 transforms the data from the spatial frequency domain to 
the spatial domain. The correlator 46 uses the complementary 
transformation to that applied by the decorrelator 30, i.e. wavelet 
coding, sub band coding or DCT coding. The output from the correlator 46 
is constant rate output video data. 
FIG. 3 illustrates a difference between the constant rate operation of the 
system of FIG. 1 and the varying rate operation of the system of FIG. 2. 
At the top of FIG. 3 there are illustrated a storage clock signal and a 
write enable signal for the constant rate system of FIG. 1. The RAM 38 is 
driven such that a byte is written into the RAM 38 when the leading edge 
of the clock signal Clk coincides with a high value of the write enable 
signal WrtEn. The constant rate output of the entropy encoder 10 is Such 
that a byte is written to the RAM 38 on every clock cycle. 
In contrast, the signals illustrated in the bottom part of FIG. 3 are from 
the varying rate system of FIG. 2. In this case, the writing of a byte of 
data to the RAM 38 again occurs when a leading edge of the clock signal 
Clk coincides with a high value of the write enable signal WPtEn. However, 
since the output of the entropy encoder 34 is at an unregulated and 
varying rate, a byte is not necessarily written to the RAM 38 upon every 
clock cycle. In general some clock cycles are missed which corresponds to 
the generally mope efficient use of the RAM 38 whereby video data of 
relatively little information content need not necessarily be evaluated to 
occupy the same RAM capacity as video data of high information content. 
It will be appreciated that whilst the present embodiment has illustrated 
the use of decorrelation, quantization, and entropy encoding as the 
compression technique, the invention is equally applicable to other 
compression techniques. Furthermore, the user adjustable compression 
control parameter of the quantization step width may be substituted by 
another control parameter within another system. 
The function of the compression control parameter is that it allows the 
user to set a particular loss of fidelity with which they wish to operate 
and then the compressed data date is allowed to vary depending upon the 
input video data information content whilst maintaining this loss of 
fidelity constant. It will also be appreciated that different sorts or, 
storage other than RAM may be used providing that they are able to handle 
variable rate writing and reading. 
Although illustrative embodiments of the invention have been described in 
detail herein with reference to the accompanying drawings, it is to be 
understood that the invention is not limited to those precise embodiments, 
and that various changes and modifications can be effected therein by one 
skilled in the art without departing from the scope and spirit of the 
invention as defined by the appended claims.