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Habermann, P., Chi, C. C., Álvarez-Mesa, M., & Juurlink, B. (2017). Syntax Element Partitioning for high-
throughput HEVC CABAC decoding. In 2017 IEEE International Conference on Acoustics, Speech and
Signal Processing (ICASSP). IEEE. https://doi.org/10.1109/icassp.2017.7952368
Habermann, P.; Chi, C. C.; Álvarez-Mesa, M.; Juurlink, B.
Syntax Element Partitioning for high-
throu
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hput HEVC CABAC decodin
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Accepted manuscript (Postprint) Conference paper |

SYNT AX ELEMENT P AR TITIONING FOR HIGH-THR OUGHPUT
HEVC CAB A C DECODING
Philipp Habermann ⋆ , Chi Ching Chi † , Mauricio Alvar ez-Mesa † and Ben J uurlink ⋆
⋆ Embedded Systems Architecture Group, T echnische Uni v ersit ¨
at Berlin, German y
† Spin Digital V ideo T echnologies GmbH, Berlin, Germany
Email: { p.habermann, b .juurlink } @tu-berlin.de, { chi, mauricio } @spin-digital.com
ABSTRA CT
Encoder and decoder implementations of the High Ef-
ficiency V ideo Coding (HEVC) standard hav e been subject
to many optimization approaches since the release in 2013.
Ho we ver , the real-time decoding of high quality and ultra
high resolution videos is still a very challenging task. Espe-
cially entropy decoding (CAB A C) is most often the through-
put bottleneck for very high bitrates. Syntax Element Parti-
tioning (SEP) has been proposed for the H.264/A VC video
compression standard to address this issue and the limitations
of other parallelization techniques. Unfortunately , it has not
been adopted in the latest video coding standard, although it
allo ws to multiply the throughput in CAB A C decoding.
W e propose an improv ed SEP scheme for HEVC CAB A C
decoding with eight syntax element partitions. Experimental
results sho w throughput improv ements up to 5.4 × with ne g-
ligible bitstream ov erhead, making SEP a useful technique to
address the entropy decoding bottleneck in future video com-
pression standards.
Index T erms — HEVC, H.265, CAB A C, Parallelization
1. INTR ODUCTION
High Ef ficiency V ideo Coding (HEVC, [1]) is the most recent
video coding standard de veloped by the Joint Collaborati ve
T eam on V ideo Coding (JCT -VC). It allows the compression
of videos with the same percepti v e quality as its predecessor
H.264/A VC [2] while requiring only half the bitrate. Context-
based Adapti ve Binary Arithmetic Coding (CAB A C, [3]) is
the entropy coding module in the HEVC standard and the
main throughput bottleneck for high bitrates because the se-
quential algorithm makes parallelization v ery challenging.
Many optimization approaches ha ve been implemented
to improv e the throughput of the critical CAB A C decod-
ing. First of all, two high-le vel parallelization techniques
ha ve been adopted in the HEVC standard, as it was not only
designed for high compression rates b ut also for high through-
put. By using T iles, a frame is split into multiple rectangular
areas that can be decoded simultaneously . W av efront Parallel
Processing (WPP) allo ws the parallel decoding of consecuti ve
ro ws of Coding T ree Units (CTUs) in the same frame. Both
techniques require the replication of the complete CAB A C
decoding hardware. T iles lead to a decreased compression
rate which is proportional to the number of tiles, because
there cannot be any inter -tile dependencies. The use of WPP
af fects the CAB A C learning process as the context v ariables
are reset at the beginning of e very CTU ro w . Ho we ver , the
coding losses are minimal for high resolution videos. Fur -
thermore, WPP has scalability issues as there is a ramp-up
and -do wn in acti ve parallel threads due to the delayed decod-
ing start of consecuti ve CTU ro ws. Overlapped W av efront
Processing (O WF) has been proposed by Chi et al. [4] as an
implementation optimization that extends WPP to multiple
parallel frames. This a voids the ramp-up/do wn phase in e v-
ery frame and scales to many more parallel threads. T iles
and WPP are not mandatory , which means that they can only
be used for improv ed decoding throughput when they were
enabled in the encoding process.
There are also lo w-le vel parallelization approaches for
CAB A C hardware decoding. Pipelining can be used to
ov erlap the decoding of consecuti v e binary symbols (bins).
Among others, this has been implemented by Chen and Sze
who used a fi ve-stage pipeline [5]. It is also possible to de-
code multiple bins per clock c ycle (e.g. Lin et al. [6] or Kim
and Park [7]). Unfortunately , the ef ficient implementation of
both techniques is limited to fe w parallel bins due to strong
data and control dependencies.
T o address the drawbacks of the described parallelization
approaches, Sze et al. ha v e proposed Syntax Element Par -
titioning (SEP , [8]) for H.264/A VC. P arallelism is exploited
by distrib uting syntax elements among dif ferent partitions, so
that they can be decoded simultaneously . This enables a sig-
nificant decoding speed-up with only minimal losses in cod-
ing ef ficiency . As only parts of the decoding hardware need
to be replicated, there is only a 50 % increase in hardw are
cost for fi ve parallel partitions. This proposal requires a mod-
ification of the bitstream format and is therefore not compli-
ant with the H.264/A VC standard. Ho we ver , the multiplica-
tion of the decoding throughput with minimal coding losses
and moderate hardware requirements mak es SEP a promising
1

Control
Luma
Chroma
(a) sequential
Control
Luma
Chroma
(b) parallel
Fig. 1 . Decoding of syntax element partitions
candidate for adoption in future video compression standards.
In this paper we present an improv ed SEP scheme for
HEVC CAB A C decoding. Section 2 describes the gen-
eral SEP functionality and the implementation of our SEP
scheme. Experimental results for decoding speed-up and bit-
stream ov erhead are presented in Section 3. Finally , the work
is concluded in Section 4 and an o vervie w of future work is
provided.
2. SYNT AX ELEMENT P AR TITIONING
Syntax element partitioning aims to di vide a common bit-
stream in multiple parts that can be decoded in parallel. Fig-
ure 1 illustrates the ef fect by sho wing the decoding process
for three groups of syntax elements. In the example, there is
one for luma and one for chroma transform blocks, as well as
a control group that contains all remaining syntax elements,
e.g. for prediction modes, prediction units and loop filters.
In a common HEVC bitstream, all syntax elements are coded
consecuti vely in a single partition, which mak es their sequen-
tial decoding necessary (a). Ho we v er , if they are distrib uted
among dif ferent partitions, parallel decoding is possible (b).
Luma and chroma transform blocks are completely indepen-
dent from each other . Their decoding process can be started
as soon as the corresponding control block is decoded. At
the same time, the decoding of the next control block can be
initiated. This allo ws the o verlapped decoding of all three
partitions. As a result, less time is required to decode all
partitions. The resulting video is the same as with the cor -
responding sequential bitstream as the same syntax elements
are only distrib uted in a dif ferent way .
2.1. Implementation
The proposed SEP scheme consists of eight partitions. First,
the common bitstream is di vided into three parts according to
the example in Figure 1: control, luma and chroma. Each of
these partitions is further split into separate parts for context-
coded (cc) and bypass-coded (bc) bins. The latter are coded
without context models, which simplifies the decoding pro-
cess. In f act, a bc bin corresponds to a bit and does not need
to be encoded or decoded at all, if it is not interlea ved with
cc bins in a common bitstream. This allo ws the highly par -
allel retrie v al of bc bins as they only need to be read from
memory . Unfortunately , the Luma and Chroma CC Partitions
still contain significantly more bins than others. T o achie ve
a more balanced distrib ution, these partitions are di vided into
two parts that contain the syntax elements for the significance
map and the coef ficient le vel. All other bins are mo ved to the
Control CC Partition.
A further split into partitions for both chroma components
is not gainful as the y use the same context models, thus mak-
ing their parallel decoding impossible. In contrast to the pro-
posal of Sze et al. we use a static partitioning scheme which
does not adapt to video characteristics. A dynamic scheme al-
lo ws a balanced distrib ution of bins to the partitions for all test
sequences. Ho we ver , the Luma/Chroma Significance Map
Partitions most often contain the majority of bins for high bi-
trates, so the maximum speed-up is determined by these par-
titions. As they cannot be split further , a dynamic partitioning
would not lead to a higher speed-up. On the other hand, the
corresponding decoding hardware can be simplified for static
partitions. The decoding of lo w bitrate videos is most often
dominated by the size of the Control CC Partition and does
not benefit from this static partitioning. Ne vertheless, their
throughput requirements are very lo w , so that real-time de-
coding is possible e ven without the use of SEP . An ov ervie w
of the proposed distrib ution among syntax element partitions
is provided in T able 1. It should be noted that some syntax
elements appear in more than one partition as they consist of
cc and bc bins. Also the same syntax elements exist for luma
and chroma transform blocks.
2.2. Bitstream Overhead
The ability to decode multiple bitstream partitions in parallel
comes at the cost of additional bitstream o verhead. First, there
is a v ariable-sized length field for e very partition (1-4 bytes)
to signal the starting position of the next partition. Addition-
ally , there is an arithmetic coding overhead for each of the fi ve
cc partitions (2 bytes). Finally , byte alignment bits are added
to all partitions (3.5 bits on a verage). This adds 16-47 bytes
of additional bitstream size per slice. The relati ve o verhead
depends on the bitrate of the video and can be significant for
very lo w bitrates. SEP can be disabled for these videos with a
single bit in the sequence parameter set or the slice header as
CAB A C decoding is usually not critical in these cases.
3. EV ALU A TION
The HEVC reference software [9] has been modified to en-
code and decode bitstreams according to the proposed SEP
scheme. Furthermore, a cycle-accurate architectural model of
2

Partition Syntax elements
Control CC
end of slice segment flag, end of subset one bit, sao merge left flag, sao merge up flag, sao type idx luma,
sao type idx chroma, split cu flag, cu transquant bypass flag, cu skip flag, pred mode flag, part mode,
pcm flag, prev intra luma pred flag, intra chroma pred mode, rqt root cbf, merge flag, merge idx,
inter pred idc, ref idx l0, mvp l0 flag, ref idx l1, mvp l1 flag, split transform flag, cbf luma, cbf cb, cbf cr ,
abs mvd greater0 flag, abs mvd greater1 flag, cu qp delta abs, cu chroma qp of fset flag,
cu chroma qp offset idx, log2 res scale abs plus1, res scale sign flag, transform skip flag, explicit rdpcm flag,
explicit rdpcm dir flag, last sig coeff x prefix, last sig coeff y prefix, coded sub block flag
Control BC
sao type idx luma, sao type idx chroma, sao offset abs, sao of fset sign, sao band position, sao eo class luma,
sao eo class chroma, part mode, mpm idx, rem intra luma pred mode, intra chroma pred mode, merge idx,
ref idx l0, ref idx l1, abs mvd minus2, mvd sign flag, cu qp delta abs, cu qp delta sign flag,
last sig coef f x suf fix, last sig coef f y suf fix
Luma Sig Map sig coeff flag
Luma Coef f Le vel coef f abs le vel greater1 flag, coeff abs le vel greater2 flag
Luma BC coef f sign flag, coef f abs le vel remaining
Chroma Sig Map sig coeff flag
Chroma Coef f Le vel coef f abs le vel greater1 flag, coeff abs le vel greater2 flag
Chroma BC coef f sign flag, coef f abs le vel remaining
T able 1 . Syntax element partitions (CC: context-coded, BC: bypass-coded)
the corresponding hardware decoder has been implemented to
estimate the maximum speed-up that can be achie ved with the
parallel decoding. T o cov er a wide range of video sequences,
the follo wing JCT -VC test sets are used for ev aluation.
• Common test conditions (class A-E) [10]
• Natural content coding conditions for HEVC range ex-
tensions (YCbCr 4:2:2, YCbCr 4:4:4, RGB 4:4:4) [11]
They are encoded in all-intra (AI), random-access (RA)
and lo w-delay (LD) modes with quantization parameters (QP)
from 12 up to 37 (common test set only specified for QP 22 to
37). In general, higher QPs result in lo wer bitrates and lo wer
video quality . The presented results are the geometric means
of all test sequences of a specific class.
The remaining e v aluation section cov ers the speed-up and
bitstream ov erhead resulting from the implementation of the
proposed SEP scheme.
3.1. Speed-up
The parallel decoding of multiple syntax element partitions
reduces the processing time and results in a speed-up (see
Figure 2). The most significant improv ements can be reached
for AI sequences. They require the highest bitrates as the y
go without the ef fecti ve inter -picture prediction. Smaller QPs
also raise the speed-ups as the resulting increased bitrates lead
to a more balanced distrib ution of bins among the dif ferent
partitions. For v ery lo w bitrate sequences, the Control CC
Partition contains most bins and determines the o verall decod-
ing throughput. Furthermore, the fraction of bc bins gro ws
with decreasing QPs. This also impro ves the throughput as
they can be decoded in a highly parallel w ay .
For all high bitrate sequences from the common test set
(Figure 2 a), the Luma CC P artition is the decoding bottle-
neck. The maximum speed-up for a single sequence is 3.8 × .
This is a significant improv ement compared to the implemen-
tation of Sze et al. who reached up to 2.3 × speed-up for high
bitrates. The sequences from the range extensions test set
(Figure 2 b) allo w an e ven better distrib ution of bins among
the partitions due to the reduced chroma subsampling. 4:2:2
subsampling results in the best balanced partitions, while the
decoding of 4:4:4 sequences is dominated by the size of the
Chroma CC Partition. The result is a maximum speed-up of
5.4 × for a single test sequence.
3.2. Bitstream Overhead
The partitioning of the bitstream for the purpose of parallel
decoding comes at the cost of additional bitstream ov erhead
(see Figure 3) as described in Section 2.2. In general, the
ov erhead depends strongly on the bitrate. This means in rela-
ti ve terms that AI videos add less bytes to the bitstream than
RA and LD videos. Also, lower QPs relati vely add less bytes.
Except for the very lo w bitrate videos in LD mode or with
high QPs, the ov erhead is less than one per cent and therefore
negligible. This is especially true for the range e xtensions test
set. SEP can be disabled for videos where it results in a sig-
nificant ov erhead as their throughput requirements are v ery
lo w .
There is one abnormal v alue in the results, because a sin-
gle test sequence (DucksAndLegs) has 60 × more o v erhead
than the other sequences from the RGB 4:4:4 class when
encoded in AI mode with QP 12. The reason is that there
are many zero bytes in one of the partitions. According to
the HEVC standard, an emulation pr e vention 3 byte is al-
3

QP22 QP27 QP32 QP37
1 x
2 x
3 x
4 x
5 x
speed-up
a) Common test set
class A class B class C
class D class E
all-intra random-access lo w-delay
QP12 QP17 QP22 QP27 QP32 QP37
1 x
2 x
3 x
4 x
5 x
speed-up
b) Range extensions test set
YCbCr 4:2:2 YCbCr 4:4:4 RGB 4:4:4
all-intra random-access lo w-delay
Fig. 2 . Speed-up
QP22 QP27 QP32 QP37
0 . 001
0 . 01
0 . 1
1
10
ov erhead in %
a) Common test set
class A class B class C
class D class E
all-intra random-access lo w-delay
QP12 QP17 QP22 QP27 QP32 QP37
0 . 001
0 . 01
0 . 1
1
10
ov erhead in %
b) Range extensions test set
YCbCr 4:2:2 YCbCr 4:4:4 RGB 4:4:4
all-intra random-access lo w-delay
Fig. 3 . Bitstream o verhead
ways added after tw o consecuti ve zero bytes. This beha vior
depends on the video characteristics and cannot be a voided.
Ho we ver , the resulting o verhead of the specific sequence is
still only 0.024% and therefore negligible.
4. CONCLUSIONS
W e hav e presented a Syntax Element Partitioning scheme for
HEVC CAB A C decoding. Bins of dif ferent syntax elements
are distrib uted among eight partitions to enable their paral-
lel decoding. As a result, a speed-up of up to 5.4 × can be
achie ved with ne gligible bitstream o verhead. The o verhead
can exceed 5 % for v ery lo w bitrates, howe ver , SEP can be
disabled for these sequences because the very lo w through-
put requirements e ven allo w sequential real-time decoding.
The proposed optimization is most ef fecti ve for high bitrates
where CAB A C decoding throughput is most critical for the
ov erall decoding performance, thus making it a reasonable
choice for adoption in future video compression standards.
Future work will co ver the implementation of the corre-
sponding hardware decoder for the proposed SEP scheme. An
additional speed-up is expected as the clustering of the com-
mon decoder will result in multiple faster decoders for the
dif ferent partitions due to smaller state machines and conte xt
model memories. Furthermore, a higher le vel of customiza-
tion can be achie ved due to the specialized operation of the
decoders for the fixed syntax element partitions. W e expect
that the parallel CAB A C hardware decoder will consume less
than 2 × the hardware resources of a sequential decoder be-
cause only parts need to be replicated.
4

5. REFERENCES
[1] G. J. Sulliv an, J. Ohm, W .-J. Han and T . W iegand,
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5

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