Process for coding and decoding stereophonic spectral values

Details Process for coding and decoding stereophonic spectral values Process for coding and decoding stereophonic spectral values

1999-05-28

2004-08-03

Harvey, Minsun Oh (Department: 2644)

Electrical audio signal processing systems and devices

Binaural and stereophonic

Broadcast or multiplex stereo

C704S219000

Reexamination Certificate

active

06771777

ABSTRACT:

FIELD OF THE INVENTION
The invention relates to coding and decoding of stereo audio spectral values, and particularly to indication of the fact that stereo intensity coding is active.
BACKGROUND ART AND DESCRIPTION OF PRIOR ART
The most advanced audio coding and decoding processes, operating e.g. to the MPEG Layer 3 standard, can compress the data rate of digital audio signals e.g. by a factor of twelve without markedly lowering their quality.
Apart from a great coding gain in the individual channels, e.g. the left channel L and right channel R, the relative redundancy and irrelevance of the two channels are also utilised in the case of stereo. The known methods which have already been used are the so-called MS stereo process (MS=centre-side) and the intensity stereo process (IS process).
The MS stereo process, which is known in the art, substantially utilises the relative redundancy of the two channels, with a sum of the two channels and a difference between them being to calculated, then transmitted as modified channel data for the left and right channel respectively. That is to say, the MS stereo process: has a precisely reconstructing action.
Unlike the MS stereo process, the intensity stereo process chiefly makes use of stereo irrelevance. It should be mentioned in connection with stereo irrelevance that the spatial perception of the human hearing system depends on the frequency of the audio signals perceived. At low frequencies both magnitude information and phase information in the two stereo signals is evaluated by the human hearing system, and perception of high frequency components is based mainly on analysis of the energy-time envelopes of both channels. Thus the exact phase information in the signals in both channels is not relevant to spatial perception. This feature of human hearing is utilised to make use of the stereo-irrelevance for further data reduction of audio signals by the intensity stereo process.
As the stereo intensity process cannot resolve precise local information at high frequencies, it is possible to transmit a joint energy envelope for both channels instead of two separate stereo channels L, R, from an intensity frequency limit defined in the encoder. In addition to the joint energy envelope roughly quantised direction information is also transmitted as side information.
As a channel is only partially transmitted when intensity stereo coding is used, the saving of bits may be up to 50%. It should be noted however that the IS process does not have a precisely reconstructing action in the decoder.
In the IS process hitherto employed in the MPEG standard, Layer 3, the fact that the IS process is active in a block of stereo-audio spectral values is indicated by a so-called mode_extension_bit, and each block has a mode_extension_bit assigned to it.
A theoretical representation of the known IS process is given in FIG.
1
. Stereo-audio spectral values for a channel L
10
and a channel R
12
are totalled at a summation point
14
to obtain an energy envelope I=L
i
+R
i
for the two channels. L
i
and R
i
here represent the stereo-audio spectral values of the respective channels L and R in any scale factor band. As already mentioned, use of the IS process is only permissible above a certain IS frequency limit, in order to avoid inserting coding errors into the stereo-audio spectral values coded. The left and right channels therefore have to be coded separately within a range from 0 Hz to the IS frequency limit. The IS frequency limit as such is determined in a separate algorithm which does not form part of the invention. From this frequency limit upwards the encoder codes the total signal of the left channel
10
and right channel
12
, formed at the summation point
14
.
Scaling information
16
for channel L and scaling information
18
for channel R are necessary for decoding in addition to the energy envelope, i.e. the total signal of the left and right channels, which may e.g. be transmitted in the coded left channel. Scale factors for the left and right channels are transmitted in the intensity stereo process as implemented e.g. in MPEG Layer 2. However it should be mentioned here that, in the IS process in MPEG Layer 3 for IS-coded stereo-audio spectral values, intensity direction information is transmitted only in the right channel, and the spectral values are decoded again with this information as explained below.
The scaling information
16
and
18
is transmitted as side information in addition to the coded spectral values of channel L and channel R. A decoder delivers audio signal values decoded in a decoded channel L′
20
and a decoded channel R′
22
, and the scaling information
16
for channel R and
18
for channel L is multiplied by the decoded stereo-audio spectral values for the respective channels in an L multiplier
24
and an R multiplier
26
, as a means of decoding the originally coded stereo-audio spectral values.
Before IS coding is applied above a certain IS frequency limit or MS coding below that limit the stereo audio spectral values for each channel are grouped into so-called scale factor bands. The bands are adapted to the perception properties of the hearing system. Each band may be amplified with an additional factor, the so-called scale factor, which is transmitted as side information for the particular channel and which constitutes part of the scaling information
16
and
18
in FIG.
1
. These factors are responsible for the formation of an interfering noise which is introduced by quantisation, in such a way that it is “masked” in respect of psycho-acoustic aspects and thus becomes inaudible.
FIG. 2
a
shows a format of the coded right channel R, used e.g. in an MPEG Layer 3 audio coding process. Any further mention of intensity stereo coding will relate to the MPEG layer 3 standard process. The individual scale factor bands
28
, into which the stereo audio spectral values are grouped, are shown diagrammatically in the first line of
FIG. 2
a.
In
FIG. 2
a
these bands are shown equal in width purely for clarity; in practice their widths will not be equal, owing to the psycho-acoustic properties of the hearing system.
The second line of
FIG. 2
a
contains coded stereo audio spectral values sp, which are non-zero below an IS frequency limit
32
; the stereo audio spectral values in the right channel above the IS frequency limit are set to zero (zero_part) nsp, as already mentioned (nsp=zero spectrum).
The third line of
FIG. 2
contains part of the side information
34
for the right channel. The part of the information
34
shown firstly comprises the scale factors skf for the range below the IS frequency limit
32
and the direction information rinfo
36
for the range above the frequency limit. The direction information is used to ensure rough local resolution of the IS coded frequency range in the intensity stereo process. Thus the direction information rinfo
36
, also referred to as intensity positions (is_pos), is transmitted in the fight channel instead of the scale factors. It should be mentioned again that the scale factors
34
corresponding to the scale factor bands
28
are still present in the right channel below the IS frequency limit. The intensity positions
36
indicate the perceived stereo imaging position (the ratio of left to right) of the signal source within the respective scale factor bands
28
. In each band
28
above the IS frequency limit the decoded values of the stereo audio spectral values transmitted are scaled by the MPEG Layer 3 process, with the following scaling factors k
L
for the left channel and k
R
for the right one:
k
L
=is_ratio/(1+is_ratio)  (1)
and
k
R
=1/(1+is_ratio)  (2)
The equation for is_ratio is as follows:
is_ratio=tan (is_pos·Π/12)  (3)
The value is_pos is quantised with 3 bits, only the values from 0 to 6 being valid position values. The left and right channels can be derived from the I signal (I=L
i
+R
i
) in the following two equations:
R
i

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