Quantization method for bit rate transcoding applications

Details Quantization method for bit rate transcoding applications Quantization method for bit rate transcoding applications

2001-01-31

2002-06-18

Wamsley, Patrick (Department: 2819)

Coded data generation or conversion

Digital code to digital code converters

To or from bit count codes

C341S067000, C375S240270

Reexamination Certificate

active

06407681

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a method of and a corresponding device for transcoding a primary coded signal previously quantized with a first quantization scale into a secondary coded signal quantized with a second quantization scale, said method of transcoding comprising at least the steps of:
partially decoding the primary coded signal, using the first quantization scale and providing a dequantized signal including DCT coefficients,
quantizing the dequantized signal, using quantization matrices and the second quantization scale and providing a quantized signal,
encoding the quantized signal so as to obtain the secondary coded signal.
The invention also relates to a corresponding device for carrying out such a method of transcoding.
This invention is particularly relevant for the transcoding of MPEG compressed video signals.
BACKGROUND OF THE INVENTION
Bit rate transcoding is a technique which allows a primary video stream encoded at a bit rate D
1
to be converted into a secondary video stream encoded at a bit rate D
2
lower than D
1
, the bit rate reduction being performed in order to meet requirements imposed by the means of transport during broadcasting. A possible method of transcoding is described in the document entitled “Seamless Concatenation—A 21st Century Dream”, by M. J. Knee and N. D. Wells, and disclosed at the International Television Symposium (Montreux, Switzerland, Jun. 13-17, 1997). Said document presents a transcoder, which is depicted in
FIG. 1
of the present invention. Such a transcoder comprises, in series, a variable length decoder (VLD), an inverse quantizer (IQ), a quantizer (Q), a variable length encoder and a buffer (VLC/BUF), and a rate control device (REG).
Unfortunately, such a transcoding method from a bit rate D
1
to a bit rate D
2
often leads to a substantial degradation of the picture quality, especially when the transcoded sequence of pictures is compared with the same sequence directly encoded at a bit rate D
2
, blurring effects or blocking artifacts being far more conspicuous on the transcoded sequence of pictures.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a method of transcoding and a corresponding device which lead to a minimal degradation in picture quality.
To this end, the method of transcoding according to the invention is characterized in that it comprises a quantization step that uses correction matrices in order to cancel undesired DCT coefficients of the dequantized signal.
The use of correction matrices comes from the following observations. The primary signal has been encoded by a video encoder at a high bit rate D
1
. As a result of this high bit rate, small quantization scales are used, leading to a lot of small DCT coefficients equal to 1 or −1. As a consequence, when quantizing, the encoder does not favor the cancellation of the small DCT coefficients especially in the high frequency domain, as shown in
FIG. 2
a
. If the primary signal is encoded at a bit rate D
2
lower than D
1
, most of the small high frequency DCT coefficients are set to 0, as shown on
FIG. 2
b
, due to the use of higher quantization scales. However, when the primary signal is transcoded from the bit rate D
1
to the bit rate D
2
, most of the small DCT coefficients of the primary coded signal remain ‘1’ or ‘−1’ after the re-quantization process, as shown in
FIG. 2
c
. These coefficients are then undesired since they do not contribute to the picture quality. On the contrary, these coefficients are encoded using the escape coding defined by the MPEG-2 standard, which is bit consuming. For a constant bit rate encoding, the bits really allocated to the picture quality are consequently decreased, resulting in an increase of the quantization scale and leading to a degradation of the picture quality. The aim of the correction matrices is to correct this negative effect, which is due to the rounding process of the re-quantization step.
The method of transcoding according to the invention is also characterized in that the quantization step comprises a sub-step of testing the neighborhood of the DCT coefficients in order to determine if the correction matrices have to be applied or not.
Such a testing sub-step allows the correction matrices to be applied in a more efficient way, leading to an improvement of the quality of the transcoded pictures.
These and other aspects of the invention will be apparent from and will be elucidated with reference to the embodiments described hereinafter.


REFERENCES:
patent: 5537440 (1996-07-01), Eyuboglu et al.
patent: 5729293 (1998-03-01), Keesman
patent: 5870146 (1999-02-01), Zhu
patent: 6201844 (2001-03-01), Bailleul
patent: 6226328 (2001-05-01), Assuncao
patent: 0944261 (1999-03-01), None
patent: WO9934602 (1998-12-01), None
Seamless Concatenation—A 21stCentury Dream, M.J.Knee, Snell & Wilcox Limited, United Kingdom, N.D.Wells BBC Research & Development, United Kingdom.

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