4 designs tips for AVB in-car infotainment

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AVB is clearly the choice of several automotive OEMs, says Gordon Bechtel, CTO, Media Systems, Harman Connected Services.

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Audio Video Bridging (AVB) is a well-established standard for in-car infotainment, and there is a significant amount of activity for specifying and developing AVB solutions in automobiles. The primary use case for AVB is interconnecting all devices in a vehicle’s infotainment system. That includes the head unit, rear-seat entertainment systems, telematics unit, amplifier, central audio processor, as well as rear-, side- and front-view cameras.

The fact that these units are all interconnected with a common, standards-based technology that is certified by an independent market group — AVnu — is a brand new step for the automotive OEMs. The AVnu Alliance facilitates a certified networking ecosystem for AVB products built into the Ethernet networking standard.

According to Gordon Bechtel, CTO, Media Systems, Harman Connected Services, AVB is clearly the choice of several automotive OEMs. His group at Harman develops core AVB stacks that can be ported into car infotainment products. Bechtel says that AVB is a big area of focus for Harman.

AVB Design Considerations

Harman Connected Services uses Atmel’s SAM V71 microcontrollers as communications co-processors to work on the same circuit board with larger Linux-based application processors. The software firm writes codes for customized reference platforms that automotive OEMs need to go beyond the common reference platforms.

Based on his experience of automotive infotainment systems, Bechtel has outlined the following AVB design dos and don’ts for the automotive products:

1. Sub-microsecond accuracy: Every AVB element on the network is hooked to the same accurate clock. The Ethernet hardware should feature a time stand to ensure packet arrival in the right order. Here, Bechtel mentioned the Atmel | SMART SAM V71 MCU that boasts screen registers to ensure advanced hardware filtering of inbound packets for routing to correct receive-end queues.

2. Low latency: There is a lot of data involved in AVB, both in terms of bit rate and packet rate. AVB allows low latency through reservations for traffic, which in turn, facilitate faster packet transfer for higher priority data. Design engineers should carefully shape the data to avoid packet bottlenecks as well as data overflow.

Bechtel once more pointed to Atmel’s SAM V71 microcontrollers that provide two priority queues with credit-based shaper (CBS) support that allows the hardware-based traffic shaping compliant with 802.1Qav (FQTSS) specifications for AVB.

3. 1588 Timestamp unit: It’s a protocol for correct and accurate 802.1 AS (gPTP) support as required by AVB for precision clock synchronization. The IEEE 802.1 AS carries out time synchronization and is synonymous with generalized Precision Time Protocol or gPTP.

Timestamp compare unit and a large number of precision timer counters are key for the synchronization needed in AVB for listener presentations times and talker transmissions rates as well as for media clock recovery.

4) Tightly coupled memory (TCM): It’s a configurable high-performance memory access system to allow zero-wait CPU access to data and instruction memory blocks. A careful use of TCM enables much more efficient data transfer, which is especially important for AVB class A streams.

It’s worth noting that MCUs based on ARM Cortex-M7 architecture have added the TCM capability for fast and deterministic code execution. TCM is a key enabler in running audio and video streams in a controlled and timely manner.

AVB and Cortex-M7 MCUs

The Cortex-M7 is a high-performance core with almost double the power efficiency of the older Cortex-M4. It features a six-stage superscalar pipeline with branch prediction — while the M4 has a three-stage pipeline.  Bechtel of Harman acknowledged that M7 features equate to more highly optimized code execution, which is important for Class A audio implementations with lower power consumption.

Again, Bechtel referred to the SAM V71 MCUs — which are based on the Cortex-M7 architecture — as particularly well suited for the smaller ECUs. “Rear-view cameras and power amplifiers are good examples where the V71 microcontroller would be a good fit,” he said. “Moreover, the V71 MCUs can meet the quick startup requirements needed by automotive OEMs.”

The infotainment connectivity is based on Ethernet, and most of the time, the main processor does not integrate Ethernet AVB. So the M7 microcontrollers, like the V71, bring this feature to the main processor. For the head unit, it drives the face plate, and for the telematics control, it contains the modem to make calls so echo cancellation is a must, for which DSP capability is required.

Take the audio amplifier, for instance, which receives a specific audio format that has to be converted, filtered and modulated to match the requirement for each specific speaker in the car. This means infotainment system designers will need both Ethernet and DSP capability at the same time, which Cortex-M7 based chips like V71 provide at low power and low cost.

What is real SAM V71 DSP performance in automotive audio?

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The integrated FPU DSP (into the Cortex-M7 core) is using 2X the number of clock cycles when compared with the SHARC21489.

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Thinking of selecting an ARM Cortex-M7-based Atmel SAM V70/71 for your next automotive entertainment application? Three key reasons to consider are the clock speed of the the Cortex-M7 (300 Mhz), the integration of a floating point (FPU) DSP, and last but not least, because the SAM V70/71 has obtained automotive qualification. If you delve deeper into the SAM V70/71 features list, you will see that this MCU is divided into several versions integrating Flash: 512 KB, 1024 KB or 2018 KB. And, if you compare with the competition, this MCU is the only Cortex-M7 supporting the 2 MB Flash option, being automotive qualified and delivering 1500 CoreMark — thanks to the 300 MHz clock speed when the closest competitor only reach 240 MHz and deliver 1200 CoreMark.

In fact, what makes the SAMV70/71 so unique is its FPU DSP performance. Let’s make it clear for the beginning, if you search for pure DSP performance, it will be easy to find standard DSP chip offering much higher performance. Take the Analog Device AD21489 or Blackfin70x series, for example. However, the automotive market is not only very demanding, it’s also a very cost sensitive market as well.

Think about this simple calculation: If you select AD21489 DSP, you will have to add external flash and a MCU, which would lead the total BOM to be four to five times the price associated with the SAM V71. (Let’s also keep this AD21489 as a reference in terms of performance, and examine DSP benchmark results, coming from third party DSP experts DSP Concept.)

Before analyzing the results, we need to describe the context:

• FIR is made on 256 samples block size
• Results are expressed in term of clock cycles (smaller is better)
• All DSP are floating-point except Blackfin
• Clock cycles count is measured using Audio Weaver

To elaborate upon that even further, this FIR is used to build equalization filter — the higher Taps count, the better. If we look at the “50 Taps” benchmark results, the SAM V71 (Cortex-M7 based) exhibits 22,734 clock cycles (about three times more than the SHARC21489). Unsurprisingly, the Cortex-M4 requires 50% more, but you have to integrate a Cortex-A15 to get better results, as both the Cortex-A8 and Cortex-A9 need 30% and 40% more cycles, respectively! And when looking at standard Analog Devices Blackfin DSP, only the 70x series is better by 35%… the 53x being 30% worst.

Now, if you want to build a graphic equalizer, you will have to run Biquad. For instance, when building eight channels and six stages graphic equalizer, your DSP will have to run 48 Biquad.

Again, the context:

• Biquad is made on 256 samples block size
• Results are expressed in term of clock cycles (smaller is better)
• All DSP are floating-point except Blackfin
• Clock cycles count is measured using Audio Weaver

In fact, the results are quite similar to those of the FIR benchmark: only the Cortex-A15 and the SHARC21489 exhibits better performance. The integrated FPU DSP (into the Cortex-M7 core) is using twice the amount of clock cycles when put side-by-side with the SHARC21489. If you compare the performance per price, the Cortex-M7 integrated in the SAMV71 is 50% cheaper! Using a SHARC DSP certainly makes sense if you want to build high performance home cinema system, but if you target automotive, it’s much more effective to select a FPU DSP integrated together with Flash (512KB to 2MB) and a full featured MCU.

The Atmel SAM V71 is specifically dedicated to support automotive infotainment application, offering Dual CAN and Ethernet MAC support. Other notable specs include:

• 10/100 Mbps, IEEE1588 support
• 12 KB SRAM plus DMA
• AVB support with Qav & Qas HW support for audio traffic support
• 802.3az Energy efficiency support
• Dual CAN-FD
• Up to 64 SRAM-based mailboxes
• Wake up from sleep or wake up modes on RX/TX

Don’t forget that when looking to construct an automotive high-end radio, you still need room for Ethernet MAC and AVB support… What’s more, the SAM V71 only consume 68% of the DSP resource, leaving well enough space for both AVB and Ethernet MAC.

Interested? Explore the Atmel | SMART SAM V ARM Cortex-M7 family here. More information about the the DSP benchmark can be also found on DSP Concept’s website.  Also, be sure the detailed DSP Concept’s audio processing benchmarks.

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This post has been republished with permission from SemiWiki.com, where Eric Esteve is a principle blogger as well as one of the four founding members of SemiWiki.com. This blog first appeared on SemiWiki on May 6, 2015.