Method for compressing an audio-visual signal

Details Method for compressing an audio-visual signal Method for compressing an audio-visual signal

1999-10-06

2004-06-22

Kelley, Chris (Department: 2613)

Pulse or digital communications

Bandwidth reduction or expansion

Television or motion video signal

C375S240260

Reexamination Certificate

active

06754273

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to methods for compressing video and audio at low bit rates in general and to methods for compressing a sub-sampled MPEG video and audio in particular.
BACKGROUND OF THE INVENTION
A single channel audio signal is considered, in the art, a single dimension function of time, while a video signal is considered a two dimensional function of time. In the art, video and audio are each sampled separately, but generally, simultaneously, since they, usually, are related. Accordingly, video and audio have to be played back and displayed in a synchronous way.
Methods for compressing digital video and audio signals, as well as decompressing the compressed digital code, are known in the art. According to a family of standards, known as Motion Picture Expert Group (MPEG) such as ISO/IEC 11172 (MPEG-1) and ISO/IEC 13818 (MPEG-2), each frame or field of the original video signal, can be compressed into three main types of pictures. It is noted that a picture in MPEG can be either a video frame or a video field.
A first type is an intra-decoded picture (I-frame) which contains all of the information needed to produce a single original picture.
A second type is a predictive picture (P-frame) which includes information for producing an original video frame, based on a previous reference frame. A reference frame is an adjacent I-frame of P-frame. The size of a P-frame is typically smaller than the size of an I-frame. A third type is a bi-directional predictive (B-frame) which includes information for producing an original video frame, based on either the previous reference frame, the next reference frame or both. The size of a B-frame is typically smaller than the size of a P-frame.
Sub-sampling refers to sampling a given signal, audio or video, at a considerably low rate, lower than an optimal one, which is usually predetermined in a given standard.
For example, the human eye is not likely to detect a single frame in a visual signal which is updated 24 times or more, in a second. The human eye regards such a visual signal as continuous motion. Thus, a video sampling rate of at least 24 video samples (frames) per second provides fluent video motion.
Similarly, the human ear cannot detect high audio frequencies. Thus a sampling rate of at least 30 KHz is likely to provide an audio signal which can not be distinguished from the original, by the human ear.
Compression standards such as MPEG are usually restricted to working according to a predetermined closed list of sampling rates in video as well as audio.
For example, MPEG operates according to a video sampling rate of, generally, 25 samples (frames) per second (when operating according to a broadcasting standard such as PAL) or, alternatively, according to a video sampling rate of, generally, 29.97 samples (frames) per second (when operating according to a broadcasting standard such as NTSC). In the context of this application 30 frames per second refers to 29.97 frames per second and is used for convenience only.
MPEG audio compression can be applied to signals, which are sampled at 32 KHz, 44.1 KHz and 48 KHz MPEG-2 allows, in addition, sampling rates of 16 KHz and 22.05 KHz.
Given a set of sampling and compression parameters, lowering the bit-rate produced by the encoder degrades the quality. Methods for maximizing the ratio between quality and bit-rate for low bit-rate MPEG applications are known in the art.
One method known in the art is applicable to video compression. The method reduces the bit-rate without effecting the quality of compressed frames and is particularly suited to compressing video with little or no motion. According to the method the signal is sub-sampled before compression and therefore some of the frames are not compressed.
According to the method, a video signal is sub-sampled according to a predetermined or dynamic duty cycle.
Where this signal to be presented to an encoder, the duration of the stream at a standard video decoder would be a fraction of the original duration. To overcome this, according to this methodthe MPEG encoder is instructed to use IP encoding (no B frames) and the stream that is produced is edited after compression. A P frame is inserted in the stream in place of each discarded frame. These P frames specify that all of the information for the frame exists in the previous reference frame in the stream and are therefore relatively small. It will be noted that this method requires editing of the compressed stream. Those skilled in the art will appreciate that the edited stream will contain a complete frame set. Moreover, the stream will be smaller than a stream that is produced by a conventional encoder that is presented with a signal from which frames were discarded and replaced by duplication of the previous frame before encoding.
It will be noted that this method is not specified for audio compression.
Reference is made to
FIG. 1
which is schematic illustration of a video signal and sub-sampled compressed video, known in the art.
Video signal
1
includes fifteen original frames referenced
12
,
14
,
16
,
18
,
20
,
22
,
24
,
26
,
28
,
30
,
32
,
34
,
36
,
38
and
40
. Video signal
1
is provided according to the NTSC standard. The NTSC standard determines a frame rate of approximately 30 frames per second. Thus, video signal
1
represents one half of a second according to the NTSC standard.
According to the prior art, in a first stage, half (every other frame) of the original frames are digitized, compressed so as to produce a frame-set
50
A. In the present example, original frames
11
,
18
,
22
,
26
,
30
,
34
and
38
are not digitized.
Frame-set
50
A is an MPEG partial representation of video signal
1
, compressed according to a sub-sampling rate of half. Frame-set
50
A includes I-frames
52
A and
72
A and P-frames
56
A,
60
A,
64
A,
68
A,
76
A and
80
A. I-frames
52
A and
72
A are compressed representation of original frames
12
and
32
. P-frames
56
A,
60
A,
64
A,
68
A,
76
A and
80
A are compressed representation of original frames
16
,
20
,
24
,
28
,
36
and
40
.
It will be appreciated by those skilled in the art that if frame-set
50
A were provided to a standard MPEG decoder, the decoder would play it, frame by frame, at a rate of 30 frames per second. Thus, frame-set
50
A, which includes 8 frames, will be played for a period of time of about one quarter of a second.
The time period spanned between original frames
12
and
40
is about half a second and so should be the time period determined by I-frame
52
A and P-frame
80
A. In reality, a decoder provides each frame {fraction (1/30)} of a second and thus, the actual time period which elapses between the displaying of I-frame
52
A and P-frame
80
A is about one quarter of a second.
To overcome this problem, a second stage is performed in which a compressing controller edits the stream and adds, after each of the compressed frame, a string of bits which represents a P-frame, relating to the adjacent previous reference frame, so as to transform frame-set
50
A into frame-set
50
B.
Frame-set
50
B includes, in addition to the frames of frame-set
50
A, P-frames
52
B,
56
B,
60
B,
64
B,
68
B,
72
B and
76
B.
Accordingly, frame-set
50
B has now an identical number of frames as the original video signal
10
. A decoder, decoding frame-set
50
B, will present frame-set
50
B in half a second, since it includes 15 frames wherein each is displayed in {fraction (1/30)} of a second.
At first, the decoder decodes I-frame
52
A and provide it for display. Then, the decoder decodes P-frame
52
B, which is a prediction that the present frame is identical to the previous one and so, the decoder provides frame
52
A for display, again. Accordingly, each of the frames originated at frame-set
50
A, is provided for displayed twice, when decoding frame-set
50
B.
The disadvantages of this method are as follows:
According to the MPEG standard the size of a P-frame that contains no information other than a reference to another frame, is around 100 bit

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