H.264 AVC / MPEG-2 Entropy Encoder IP Block - SNUG (Slice NAL Unit Generator)

The SNUG is an entropy encoder and (optional) associated DMA engines that reads swathes of macroblock coefficients and information from memory and writes an encoded bit stream to a circular memory buffer.

H.264 AVC capability

The H.264 standard does not formally define profiles or levels as a way to describe encoders, since these terms refer to the characteristics of a bitstream and the capability of a decoder implementation. However, broadly speaking, the SNUG is capable of encoding bitstreams conforming to the Main, High and High 4:2:2 profiles:

The SNUG does not support the encoding of bitstreams conforming to the Baseline, Extended, and High 10 profiles, and does not support any scalable coding profiles. The SNUG is capable of producing both CAVLC and CABAC bitstreams and supports the production of byte streams conforming to Annex B of H.264.

Restrictions on H.264 syntax and semantics

· The SNUG is designed to support a simple raster scan ordering of macroblocks in which a slice encompasses a number of whole rows of macroblocks.
· The SNUG restricts the luma and chroma samples to have the same depth as one another.
· The SNUG supports reference picture indexes in the range zero to fourteen.
· PCM is not supported.
· Macroblock adaptive frame-field coding (MBAFF) is not supported when the entropy in CAVLC mode. MBAFF is supported in CABAC mode.

H.264 performance levels

Assuming a clock rate of 500MHz or greater SNUG supports level 4.2, and all lower levels, of the Main, High and High 4:2:2 profiles (i.e. 1920x1080x60P). The SNUG itself is capable of supporting bit-rates up to roughly 250Mbit/s, although this is highly dependent on factors outside the control of the SNUG. (For example, if all transform coefficients were zero then the bit-rate would be very low, and limited by the input DMA.)

MPEG-2 capability

The MPEG-2 video standard does not formally define profiles or levels as a way to describe encoders, since these terms refer to the characteristics of a bitstream and the capability of a decoder implementation. However, broadly speaking, the SNUG is capable of encoding bitstreams conforming to Main and 4:2:2 profiles:

Restrictions on MPEG-2 syntax and semantics

The SNUG does not support:

· Encoding of MPEG-1 video bitstreams. (This may have been expected since a compliant MPEG-2 decoder is required to decode MPEG-1 bitstreams)
· Simple Profile.
· Multiple slices in a row of macroblocks.
· Encoding pictures whose vertical size is greater than 2800 lines.
· Encoding of concealment motion vectors.
· Dual Prime motion vectors.
MPEG-2 performance levels

Assuming a clock rate of 500MHz or greater SNUG supports the High level, and all lower levels, of the Main, High, and 4:2:2 profiles (i.e. broadly 1920x1080x30). The SNUG also supports coding at up to twice the pixel rate required of High Level (i.e. broadly 1920x1080x60 – there is no MPEG-2 Level defined to describe this capability). The SNUG itself is capable of supporting MPEG-2 at bit-rates up to roughly 250Mbit/s, although this is highly dependent on factors outside the control of the SNUG. In addition, in a system the performance will be lower because memory accesses are unlikely to be serviced immediately due to contention with other memory clients.

For more information on SNUG please click here.

The Aspex Q Semiconductor IP (SIP) Compression Engine is available for license as a stand-alone block, which can be integrated into a custom ASIC or ASSP implementation for specific applications, such as:

• Wireless HDMI chipsets
• PC Graphics chipsets
• HD Camcorders
• Digital Still Cameras
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Software running in the Q's Control Unit can implement a wide range of codecs, including

• H.264 / AVC (Baseline, Main and/or High Profiles)
• Scalable Video Codec (SVC)
• MPEG-2
• MPEG-4 Part 2 (Advanced Simple Profile)
• AVS
• Flash (On2 VP6)
• Motion JPEG
• JPEG-2000

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To find out more about licensing the Aspex Compression Engine for your application, please contact us directly.