Introduction

The number of smartphones that have eliminated the earphone jack is increasing, and audio listening using Bluetooth has become more common. I think many people actually use Bluetooth earphones for commuting.

The codecs used in Bluetooth audio transmission are also progressing, and high-quality codecs are also appearing. Qualcomm provides Qualcomm® aptX™ as a high-quality codec for Bluetooth audio.

What is a Bluetooth codec

The maximum throughput of Bluetooth (the maximum amount of data that can be transmitted in one second) is said to be 1Mbps (1Mbit/s = 125kbytes/s) in an ideal environment (environment that does not interfere with radio such as Wi-Fi). The throughput required to transfer the CD sound source as it is is 44.1 kHz × 16 bit × 2 channels = 1411200 bps (≒ 1.411 Mbps > 1 Mbps), so it cannot be transferred with the transmission capacity of Bluetooth as it is.

Therefore, Bluetooth compresses audio data in order to transmit audio stably, and codecs are used in that audio compression technology. It's the same principle as a long time ago when music was compressed in MP3 and carried on an audio player with a small capacity (MP3 is also one of the codecs, although it is not used in Bluetooth).

Features of Traditional Bluetooth Codec (SBC)

SBC (SubBand Codec), which is a standard codec for A2DP (Advanced Audio Distribution Profile), a profile for Bluetooth audio transfer, and a codec that must be supported, has been used for a long time. One of the parameters) is greatly affected by the codec.

Older products or products that do not have sufficient transmission capacity use lower Bitpool values to lower the bitrate (amount of data sent per second), so connecting with such products may result in higher Bitpool values. Even if you are using relatively new earphones or headphones that are compatible, the low Bitpool value is used, so the sound quality is degraded. Bitpool values are generally invisible values (values that cannot be confirmed without using specialized equipment), which is also a factor that confuses users.

In addition, since the masking process (thinning out the inaudible range during compression) using a psychoacoustic model that is performed when encoding to AAC or MP3 is also performed in SBC encoding, AAC or MP3 sound sources are transmitted via Bluetooth. In that case, the masking process will be performed twice, and there is also a characteristic that the sound will feel uncomfortable compared to the original sound.

Difference between Qualcomm® aptX™ and SBC

Qualcomm® aptX™ uses a unique codec method, and the major difference is that it does not use masking processing (thinning out the non-audible range) using a psychoacoustic model like SBC. Qualcomm® aptX™ uses a unique fixed compression algorithm that does not use a psychoacoustic model to achieve flat frequency characteristics that are faithful to the original sound, and this technology ensures high sound quality.

In addition, SBC is a frame-based codec (a codec that encodes sound for a certain amount of time as one frame), and since it cannot be decoded until one frame has been received, the delay is relatively large, whereas Qualcomm® aptX ™ is a sample-based codec (a codec that encodes in units of sound samples that are sampled at a sampling frequency per second), and the difference is that sample data can be decoded as soon as it is received, so there is little delay. I have.

Features of Qualcomm® aptX™ Family

Qualcomm® aptX™ offers a wide range of lineups such as Qualcomm® aptX™ / Qualcomm® aptX™ Low Latency / Qualcomm® aptX™ HD / Qualcomm® aptX™ Adaptive. Each feature is summarized below.

・CD-quality high-quality audio codec

・Technology controversial in professional audio fields such as recording studios and broadcasting stations

・Adopts a unique algorithm suitable for Bluetooth audio transmission that does not cause voice quality degradation or delay problems

・Supports sending function on major OS (Windows10, macOS, Android)

・Low-latency audio codec with no lip-sync

・Achieved a difference of 40 msec or less from the image

・Implement packet forwarding and error correction processing for low latency based on aptX

・Sound quality is equivalent to Qualcomm® aptX™

・24bit Bluetooth audio codec that supports high-resolution sound sources

・SNR 129dB（ハイレゾ相当の低ノイズ）を実現

・Adopts original packet error correction algorithm

・プロ音響分野で実績のある24bitアルゴリズムをBluetoothに採用

・Codec that combines high sound quality, low latency, and robustness (strength of sound interruption)

・Suppress skipping by automatically adjusting the bit rate according to changes in the RF environment

Bit-error tolerance helps maintain audio quality in harsh RF environments

・Dynamic shift to low-latency mode according to the use case

Qualcomm® aptX™ compatible main lineup

(*) Supports 48kHz/24bit transfer only
(**) In addition to 48kHz/24bit transfer, 96kHz/24bit transfer is also supported, and Qualcomm® Snapdragon Sound™ is also supported.
(***) In addition to 48kHz/24bit transfer, 96kHz/24bit transfer is also supported, and Qualcomm® Snapdragon Sound™ support also supports Lossless transfer.

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