Trick mode VTR which generates trick play data from a stream...

Details Trick mode VTR which generates trick play data from a stream... Trick mode VTR which generates trick play data from a stream...

1997-11-24

2002-11-12

Olms, Douglas W. (Department: 2732)

Motion video signal processing for recording or reproducing

Local trick play processing

With randomly accessible medium

C386S349000, C386S349000, C348S420100, C348S403100

Reexamination Certificate

active

06480664

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a tape format for facilitating trick mode playback, i.e., fast forward or reverse playback, of compressed video pictures or images. The present invention also relates to tracking the compressed video information during trick mode playback. Furthermore, the present invention relates to generating, from the compressed video pictures, information that is necessary for presenting the compressed pictures during trick mode playback.
BACKGROUND OF THE INVENTION
In order to understand the present invention it is useful to review the state of the art of video tape transport devices and video tape formats. It is also beneficial to review certain aspects of video compression. Both are discussed below.
Video Tape Transport and Formatting
FIG. 1
shows a conventional video tape transport and scanner assembly
10
, which scanner includes a rotary drum
14
with recording/playback heads A,B positioned thereon. (The invention is illustrated herein using a two head A,B drum
14
although the principles described herein are equally applicable to drums having a different number of heads such as four.) A video tape
12
is wrapped partially around the circumference of a drum
14
, e.g., 180° around the circumference of the drum
14
. The tape
12
is transported around the circumference of the drum
14
. As shown, the tape is transported at angle &thgr;° to a line perpendicular to the axis of rotation of the drum
14
. While the tape is transported in the indicated direction, the recording/playback heads A,B rotate in the indicated direction. As each head A or B rotates in the proximity of the portion of the tape
12
wrapped around the cylinder, the head A or B scans over a portion of the tape
12
. Each head A or B scans a portion of the tape
12
in a round robin fashion.
FIG. 2
illustrates the scanning of the heads A,B over the tape
12
in greater detail. As shown, each head A or B scans a diagonal segment of the tape
12
referred to as a track
18
. During recording, a signal (e.g., an analog composite NTSC or PAL video signal) is recorded by the heads A and B onto the tracks
18
as each head A or B scans a track
18
. (Illustratively, alternate tracks are recorded with opposite magnetic polarities, as shown by oppositely slanted diagonal lines, to reduce inter-track interference.) Likewise, during playback, the signal is reproduced from the tracks as the heads A and B scan each track
18
. The angle &thgr;
r
of the track
18
with respect to the tape axis depends on the relative transport speed of the tape
12
and the rotational speed of the drum
14
and the angle of transport &thgr;°. As may be appreciated, to reproduce a signal properly, the transport and scanner assembly
10
must cause each head A,B to substantially scan each diagonal track
18
in sequence and in relative alignment with the angle of the track
18
, as shown by the arrow
25
. To that end, an automatic tracking frequency (ATF) word is illustratively recorded on each track
18
, which ATF word is reproduced by a head A or B during playback. The ATF word of each track produces a signal with a particular frequency. As shown in
FIG. 1
, this signal is fed to a feedback circuit
22
which controls the drum servo
24
and the capstan servo
26
. The feedback circuit
22
compares the frequency of the ATF signal to a target frequency. Depending on this comparison, the relative speed of the drum
14
and the tape
12
transport is either increased or decreased to ensure that the heads A,B scan each track
18
successively.
In a conventional analog VTR, each field of video occupies an equal amount of space on the tape. In particular, each field is recorded on a single track; there is a one-to-one correspondence between tracks and fields. Thus, during playback, the scanning of one head A or B produces a video signal for presenting one field of the video on a display device.
The trick modes of concern herein are fast forward and fast reverse playback modes in which the video information is played back at a faster rate than the normal playback speed. In order to provide such fast or n×normal speed playback, only a fraction of the video information is presented on the display device. For instance, during 3×normal speed playback, only one third of the video information is presented on the display device. During fast forward or fast reverse, the relative speeds of the tape and the rotation of the heads is much faster than during normal speed playback. Thus, the heads do not scan in relative alignment to a single track but rather cross a number of tracks as illustrated by the arrow
30
in FIG.
2
. Note that each track corresponds to a single field. Furthermore, there is a correspondence between the location within a given track
18
in which a particular portion of the video information is recorded and the location of the portion of the field that the particular video information portion reproduces. Stated another way, assume the scan of a head A or B crosses the first, middle and last thirds
45
,
50
and
55
of three tracks
60
,
65
,
70
, which tracks
60
,
65
,
70
correspond to first, second and third consecutively displayed fields. This scan produces a video signal with video information for the first third of the first field, the middle third of the second field and the last third of the third field. Thus, the video signal produced by an n×normal speed playback includes a relative piece of the video signal from n different fields, each at a respective portion of the image. Because there is a large correlation from field to field, this “piece-meal” video signal can be presented as an intelligible image; the viewer will perceive a single, reasonably correlated image of low fidelity, even though the user is viewing concatenated portions from n fields.
Video Compression
Advantageously, an audio-visual presentation or program bearing signal is digitized and compressed before the video signal is recorded on the video tape. For example, the video and audio portions of an audio-video program may be compressed according to the Motion Picture Experts Group (MPEG) II recommendations. See ISO/IEC DIS 13818-2: Information Technology—Generic Coding of Moving Pictures and Associated Audio Information. The contents of this document are incorporated herein by reference. Illustratively, such encoding and storage produces a hierarchically organized signal. Furthermore, yet another layer in the hierarchy may be provided, namely, a storage or channel layer and a tape format, for formatting such a compressed MPEG II signal for physical storage on the tracks of a tape. In summary, the different layers of the hierarchy are as follows:
(1) video and audio elementary streams, which contain the raw compressed video and audio data,
(2) a transport stream (or program stream) which contains the video elementary streams, audio elementary streams and other private data (such as closed-captioned text) of one or more audio-visual programs, and
(3) a storage/channel stream, including a tape format, which encapsulates one or more transport streams with, amongst other things, error recovery data and which physically organizes the data into individual tracks on the tape.
MPEG II provides a specification for the elementary stream and transport stream layers and is believed to be a preferred way to compress and organize video and associated audio information. Therefore, this invention is illustrated using the above hierarchy and in particular, using MPEG II compliant elementary streams and transport streams. Each of these streams is discussed in greater detail below. Furthermore, because this invention is directed to trick mode playback, i.e., n×normal speed playback, only video reproduction is of concern. Therefore, audio and other non-video data is not discussed for purposes of brevity.
Video Elementary Streams
MPEG II provides for compressing video by reducing both spatial and temporal redundancy. A good tutorial for MPEG II video compression is contained in D. Le Gall,

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