aptX technology perfectly solves the delay problem of wireless audio

The aptX audio compression codec technology has completely revolutionized the listening experience of Bluetooth stereo audio, and can provide high-quality wireless audio for consumer electronic applications such as Bluetooth stereo headphones and various speakers. The aptX technology was originally used in radio broadcasting, and was not introduced into the Bluetooth application field until 4 years ago. Its application enables devices that support stereo Bluetooth A2DP connections to output CD-quality audio.

Although aptX technology overcomes the lower sound quality of Bluetooth audio, developers still need to solve the latency problem of Bluetooth audio in order to develop high-quality wireless headphones and speakers for consumers who use mobile devices to watch movies or play games. Latency refers to the time it takes for the audio stream to be transmitted from the audio source (smartphone, media player, computer or console) to be played on the wireless receiving device (speaker or headphones).

When watching movies on a tablet with wireless headphones, users don't want the sound to be out of sync with the picture; and when playing intense action games involving shooting, explosions, and other intermittent accidents on wireless devices, gamers also want intuitive audio response. In order to achieve synchronization between sound and action, the delay must be controlled within 40ms. Since the delay time of traditional Bluetooth technology exceeds 100ms, its performance cannot achieve real-time synchronization of dialogue and game. Therefore, various patent-based RF solutions (most of which require adapters) have emerged to fill the huge gap in this field, but there are very few successful cases.

CSR's recently launched low-latency aptX audio compression codec technology allows developers to easily solve latency issues without using complicated adapters. The new low-latency aptX audio compression codec technology not only provides lossless high-quality audio through Bluetooth, but also provides low-latency output performance as low as 40ms. aptX breakthrough technology has many advantages, including:

 Uses standards-based RF technology and is Bluetooth compatible
 No adapter is required as Bluetooth is 100% compatible with smartphones, tablets and laptops
 Can run on existing chip designs through a simple software update

Low-latency audio compression codec technology issues

The key to this problem lies in the advance or lag of audio and video signals. According to the European Broadcast Recommendation R37-2007, the time gap between the transmission of the sound and image components of a TV signal is: "The sound should not be more than 40ms ahead of the image signal and should not be more than 60ms behind." In stereo audio transmission, the Bluetooth transport layer uses the A2DP protocol that supports the standard SBC codec and the encoding algorithm based on psychoacoustic perception technology. However, the compression technology based on SBC and perception uses frame compression, and its overall delay time can be as high as 100ms to 500ms, which is mainly attributed to the following three factors:

1. Codec delay: Each audio codec causes a certain internal delay before encoding the data and sending it out. Traditional codecs have been detected with codec delays of up to 50ms.

2. Transmission delay: The A2DP transmission layer uses a data packet structure. When engineers use SBC-based or perception-based frames to fill data packets, they have two options: one is to put a frame into a large Bluetooth data packet; the other is to decompose a frame into two Bluetooth data packets. The first option will reduce the robustness of data transmission, while in the second option, the decoder can only decode the decomposed frame after receiving two Bluetooth data packets, which will significantly increase the transmission delay time.

3. The codec will not start decoding until it has received the entire packet or frame.

In addition, due to their bit rate setting characteristics, the audio quality provided by SBC and perceptual based codecs is highly variable, that is, even using multiple copies of the same codec will not provide stable audio performance.

A2DP transmission cannot have the best of both worlds. If you want low latency performance, then the robustness and sound quality of the audio will be reduced. Therefore, many proprietary RF solutions have been launched on the market to meet consumers' demand for wireless transmission.

aptX low latency technology

The sampling-based aptX codec has two major advantages over the A2DP codec: It provides an ultra-low encoding and decoding delay of 1.9ms, and more importantly, does not require a frame format. Therefore, during the decoding process, the aptX codec can efficiently fill the Bluetooth data packet without waiting, that is, once it receives the data packet, it will start the decoding process immediately. In addition, aptX uses a fixed compression rate algorithm, which can always provide the same bit rate during transmission, thereby ensuring that every product equipped with aptX outputs the same sound quality.

aptX's unique set of features delivers professional audio performance and robustness while guaranteeing an end-to-end latency of 40ms, which is in line with the European Broadcasting Union (EBU) standard for synchronizing dialogue and motion.

TV and games

Since the advent of flat-panel TVs, engineers have been working to narrow the TV bezels, mainly to better fit the TV on the bedroom wall. However, the thinness of a TV is inversely proportional to its sound quality, and the sound quality of its speakers is proportional to its volume, which means that the thinner the TV, the worse its sound quality. For the 5mm-thick flat-panel TVs currently on the market, we offer two low-latency Bluetooth connection solutions:

1. Manufacturers can make standalone speakers, build aptX low latency technology into the TV casing and embed the decoder in the speakers
2. TV buyers can use a receiver to transmit the signal from the line output port to an existing aptX-compatible stereo system

Gaming is also a growing application area. While the traditional indoor game console market is still strong, the mobile game market has been booming. Modern operating systems allow all smartphones and tablets on the market to run 3D games, which will create hundreds of millions of potential users for the game market.

Players who control games on the road with Bluetooth headphones definitely don't want to encounter delay issues. They hope that the sound they hear through the headphones can be synchronized with the game screen and control actions. If the time interval between pulling the trigger and hearing the gunshot exceeds 50ms, the delay will be more obvious, and the game will become boring.

aptX low latency technology and its 40ms low latency ensures gamers get the audio sync they need, and aptX can also open a two-channel audio stream to send the signal from the headset back to the game console, providing wireless voice chat for gaming teams and enhancing the gaming experience.

Headphones with built-in aptX can be used for playing games, listening to high-quality music, hands-free calls and other applications, creating a variety of wonderful experiences for users.

How to use aptX low latency technology

The two ends of the aptX low-latency technology connection are: compression/encoder/source and decompression/decoder/sink. It is suitable for various encoders based on Linux, Android Jelly Bean and Kit Kat systems on the market, and is compatible with BlueZ and Bluedroid Bluetooth protocol stacks. Once the output signal is confirmed to be accurate, data integration can be completed within two working days.

On the decoding side, users can use CSR chips such as BC05MM and SR8670, both of which are flash-based chip components compatible with Kalimba onboard DSP. Users can then integrate aptX low-latency technology into the Kalimba DSP platform. In addition to aptX, CSR chips can also provide a variety of other rich functions, including high-performance DAC and ADC, capacitive touch, input/output functions (SPDIF, I2S, I2C and GPIO, etc.); if CSR8670 is used, Bluetooth Smart connection function can also be provided.

What are your choices?

As mentioned at the beginning of this article: the limitations of SBC and other frame-based codecs over Bluetooth connections have forced the A/V industry to choose other proprietary solutions. Given that CSR technology has successfully solved the latency issue, manufacturers can now consider adopting Bluetooth connectivity in their ecosystem designs, which is not only easy to use, but also cost-effective (chip and system) and ensures that their products will never be outdated.

Abandoning proprietary solutions in favor of standards-based solutions allows manufacturers to benefit from the research and development of various Bluetooth semiconductor companies. The benefits include:
 Accelerating the design process
 Creating economies of scale to ensure competitive pricing
 Enjoying a variety of other rich features developed in addition to the Bluetooth radio, including on-board DSP, ADC, DAC, and sample rate converters

The Bluetooth standard has been adopted by a wide range of consumer electronics manufacturers, and the sales of Bluetooth chips have exceeded 3 billion in the past decade. All smartphones, tablets, most feature phones and portable media players on the market are equipped with Bluetooth functions and compatible with the A2DP protocol to provide wideband stereo audio. It will be a cause of great concern to enable users to listen to high-quality audio through wireless headphones and speakers when watching videos stored on smartphones without using electronic dongles.