Audio design in smart phones

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When the mobile phone continuously integrates various functions including photography, games, data, video, etc., it has turned into a multimedia application playback platform, which can be said to be a meticulous and compact portable mini computer. In terms of positioning, such a mobile phone is different from an existing pure voice phone or a feature phone, and belongs to a smart phone.

In addition to strong data editing and management capabilities, smart phones can provide multimedia application services such as audio, video and games, and can handle multiple jobs at the same time. Further, its functional aspects cover communication, information and multimedia functions, namely:

1. Communication function: voice, message (messaging), authentication (Authentication), billing (Billing) and other communication processing functions;

2. Information function: Email, calendar, information management, Sync, security and other information processing functions;

3. Multimedia functions: multimedia application functions such as video, camera, game, TV, streaming, music, DRM;

In addition to information functions, audio is a necessary processing task in communications and multimedia applications. In the past, mobile phones only needed to handle simple voice call signals, but today's smart phones have to deal with a lot of audio tasks. In addition to multi-tone ringing and MP3 music, there may be FM radio and game sound effects, and not just The mono effect now requires a stereo presence experience.

In the past, the world of digital audio was two-fold: one is the world of Hi-Fi, and the other is the world of speech. In general, Hi-Fi refers to 16-bit stereo quality, audio sampled at 44.1 kHz, which is the specification of CD music; telephone voice is 8-bit and 8 kHz mono, low-quality audio. However, in the era of smart phones, the two audio worlds began to collide. How to integrate the audio subsystems perfectly with application and communication processing platforms has become a key challenge for portable device engineers to develop new products. .

Audio coding format and interface

Before going into the discussion of system architecture, let's take a look at the current state of audio coding. At present, there are many formats for audio coding. For the encoding of sound, there are PCM, ADPCM, DM, PWM, WMA, OGG, AMR, ACC, MP3Pro and MP3; for human voice, there are LPC, CELP and ACELP; others have MPEG- 2. The encoding format of audiovisual programs such as MPEG-4, H.264, and VC-1. Please refer to the market trend of mobile multimedia formats (Figure 1).

Figure 1 Application market trends of mobile multimedia formats

The following describes three common audio formats:

AMR format

AMR is an adaptive multi-rate speech codec (Adpative Multi-Rate Speech Codec). The original version is the speech coding standard developed by the European Telecommunications Standards Institute (ETSI) for the GMS system, and the bandwidth is divided into two. AMR-NB (AMR Narrowband) and AMR-WB (AMR Wideband). For Nokia, the market's largest brand, most of its phones support audio files in both formats.

MP3 format

MP3 is the abbreviation of MPEG AudioLayer3, which is an audio compression technology with a high compression ratio of 10:1-12:1, which can keep the low frequency part undistorted, but sacrifices the high frequency part of 12KHz -16KHz in audio. To reduce the file size, its ".mp3" format file is generally only 10% of ".wav". In addition, one of the reasons why MP3 is popular is that it is not a copyright-protected technology, so anyone can use it.

MP3 format compressed music has a variety of sampling frequencies, can save space with 64kbps or lower encoding, can also use 320kbps to achieve extremely high compression quality. MP3 is divided into "CBR" (fixed code) and "VBR" (variable bit rate) technology in encoding rate. Some mobile phones cannot play downloaded music because there is no MP3 supporting "VBR" format. music.

AAC format

AAC is Advanced Audio Coding, which uses a different operation method than MP3. AAC can support up to 48 tracks, 15 low-frequency tracks, more sample rates and transmission rates, and multiple types. Verbal compatibility and higher decoding efficiency. In summary, AAC can provide better sound quality under the condition of 30% smaller than MP3 format, and the sound fidelity is better, closer to the original sound, so it is regarded as the best audio encoding format by the mobile phone community. AAC is a large family, they are divided into 9 specifications to suit the needs of different occasions:

(1) MPEG-2AAC LC Low Complexity Specification (Low Complexity)

(2) MPEG-2 AAC Main main specifications

(3) MPEG-2 AAC SSR Variable Sample Rate Specification (Scaleable Sample Rate)

(4) MPEG-4 AAC LC low complexity specification (LowComplexity), the audio part of the more common MP4 file of the current mobile phone includes the audio file of the specification.

(5) MPEG-4AAC Main main specification

(6) MPEG-4 AAC SSR Variable Sample Rate Specification (Scaleable Sample Rate)

(7) MPEG-4 AAC LTP Long Term Prediction Specification (Long Term Prediction)

(8) MPEG-4 AAC LD Low Latency Specification (Low Delay)

(9) MPEG-4 AAC HE High Efficiency Specification

Among the above specifications, the main specification (Main) includes all functions except the gain control, and the sound quality is the best, while the low complexity specification (LC) is relatively simple, without gain control, but the coding efficiency is improved. SSR and LC specifications are roughly the same, but the gain control function is added. In addition, LTP/LD/HE is used for encoding at low bit rate. HE is supported by NeroACC encoder, which is a commonly used coding rate method. . However, in general, the sound quality of the Main and LC specifications is not much different. Therefore, considering the current memory of the mobile phone is still limited, the most widely used AAC specification is the LC specification.

audio port

Audio interfaces are an important consideration for smartphone designers. Digital voice generally uses PCM (Pulse Code Modulation) interface, while Hi-Fi stereo uses serial I2S (Inter-IC Sound) interface or AC97 interface. I2S is a bus standard developed by Philips for audio data transmission between digital audio devices. It is a commonly used interface in consumer audio products. AC?7 is used by Intel to improve personal computer sound and reduce noise. The specification, due to its development in 1997, is called AC97.

In terms of computer audio requirements, it is basically similar to the consumer market, but in order to play music files recorded at different sampling rates (8 kHz, 44.1 kHz, 48 kHz), a more efficient and cheaper solution is needed. AC97 has such characteristics. In the generalized handheld device market, each of the three formats has its own advocates: CD, MD, MP3 players will use the I2S interface; mobile phones will use the PCM interface; audio-enabled PDAs use the same AC97 encoding as the PC. format.

Audio system integration strategy

In earlier systems, it was common to discharge the circuitry of the phone and PDA into the device enclosure, where the PCM voice code was controlled by the communications processor and the Hi-Fi stereo (AC?7 or I2S) was processed. Go to another application processor. In this architecture, the integration between the two audio subsystems is still very low, and the distributed hardware switching circuit will bring harmonics in addition to space and requires additional peripheral components for signal exchange and mixing. Problems such as harmonic distortion. Please refer to (Figure 2).

Figure 2 PCM and stereo separately processed audio architecture

Therefore, it is ideal to tailor an integrated solution for a specific application. Under the SoC technology trend, some vendors have integrated stereo digital-to-analog converters (DACs) or codecs (CODECs) into ICs with specific functions. However, some features are suitable for integration, while others may be counterproductive.

For example, when vendors integrate power management and audio processing functions, they usually have to compromise on the sound quality because the noise generated by the power regulator can interfere with nearby audio paths; The integration of functions into digital ICs is also difficult, because Hi-Fi components require a 0.35mm process to optimize the mixed-signal processing, but the current digital logic applications are below 0.18mm. Higher process development. In the above two integrated chip strategies, in order to allow two different circuits to exist in one chip at the same time, the final chip size may be unacceptably large.

In addition, speaker amplifiers are particularly difficult to integrate. The heat it generates is a problem that requires heat dissipation, so a separate speaker driver IC is often required. There is also a common problem with integration, which is to minimize the number of analog inputs or output pins in order to minimize the IC.

Dedicated audio ICs avoid these problems, and there are several ways to achieve audio integration. Sharing ADCs and DACs can reduce hardware costs, but not both audio streams can be played or recorded simultaneously. Arranging a dedicated converter for individual functions can solve this problem, but this approach increases the cost of the chip. The compromise is to share only the portion of the ADC, but with a separate DAC. In doing so, while the phone is communicating, you can play other audio at the same time (such as playing another ringtone or playing music), but You cannot record at the same time during communication. The power consumption of the ADC can be controlled by turning off a function and at a lower sampling rate. Please refer to (Figure 3) and (Figure 4).

Figure 3 Exclusive audio processing system concept

Figure 4 Intelligent mobile phone audio processing architecture

In addition, the audio system can have different approaches. When the voice CODEC is integrated into the communication chipset, it is appropriate to pair with another Hi-Fi CODEC with additional analog inputs, outputs and internal mixes; in another case, a direct-link wireless headset function The dual CODEC of the exclusive PCM interface (such as Bluetooth) also has its use benefits, please refer to (Figure 5).

Figure 5 Dual CODEC audio architecture with wireless headset capabilities such as Bluetooth

The following is a planning analysis of several important components in an audio system:

Frequency and interface

While the internal circuitry of the shared communication and application subsystem is feasible, it is not the case for interfaces because different audio applications operate at their own frequencies in separate frequency regions. As long as the situation is still the case, the CODEC of the integrated smart phone needs to have both a PCM interface and a separate I2S or AC97 link.

In non-mobile devices (such as PCs), the audio frequency is usually generated by a crystal oscillator, but in the design of smart phones, in order to avoid additional power consumption, board space and frequency chips Cost, the designer prefers to separate the frequency functions required for Hi-Fi audio from the existing frequencies. Since low-power, low-noise phase-locked loops (PLLs) can be integrated into mixed-signal chips at relatively low cost, today's chipmakers are merging one or two PLLs into their smart phone CODECs. in.

microphone

The most difficult design issues in smart phones are often related to microphones (Mic). In general, there are at least two microphones to consider: one is the built-in internal microphone and the external microphone that plugs into the headset. In addition, there may be additional internal microphones for noise cancellation or stereo recording, as well as another external microphone for the hands-free function of the car. In addition to speaking, these microphones can also record sound effects in voice messages or video clips through the control of the application processor.
To cover various switching functions by the audio CODEC chip, the circuit of this chip needs to be properly designed. In addition to the recording function, the CODEC should also provide a side tone function so that the headset user can hear his own voice. The plug-in detection feature provides seamless switching, which means that when the headset is plugged in or unplugged, the system automatically switches between using internal or external headphones.

Acoustic noise cancellation is another problem. It requires two microphones, one to receive both the speech and background noise, and the other to receive only background noise. The analog method is often not enough, so it needs to be enhanced by digital signal processing, and the audio CODEC needs to achieve the digitization task of two microphone signals.

Another problem is the problem of outdoor wind noise, which is usually less than 200 Hz, so it can be processed by a high-pass filter, but in this case, the low frequency part is lost during indoor recording. sound. For dual-purpose microphones, this filter should be optional, but many high-pass filters are built into the audio ADC, so handset manufacturers should choose a solution for their needs.

External headphones

The use of a headset (headset/headphone) also requires a special analog circuit, that is, when the headset is plugged in, the audio output signal can be routed to the head of the headset. Although a socket incorporating a mechanical switch can meet this requirement, it is oversized and expensive; in addition, the volume of the speaker may not be suitable for this headset. Providing independent volume control for internal and external audio solves this problem and allows for a simpler slot design. Whether or not the external earphone has a microphone also needs to be detected. This can be resolved by sensing whether a bias current is present. If no current flows, it means that no microphone is inserted. This current sensor should be added to the audio CODEC of the smart phone, so that the audio input and output can be processed according to different situations.

speaker

After the smartphone has added functions such as multi-tone ringing, MP3 broadcasting and FM broadcasting, its broadcasting system has also developed toward stereo speakers. In the design of mobile phone speakers, the main problem is the configuration architecture, power and power consumption considerations. To support stereo, the phone needs to have two external speakers, but because the size of the phone is too small, the position of the two speakers is difficult to pull apart, so the stereo effect is not easy to display, then special 3D effect processing is needed. If you want to support the speakerphone function, you need to connect to another large speaker. Providing a dedicated analog output for individual speakers is the best way, but there must be a corresponding change in power management.

Since the speaker power amplifiers use a lot of power, it is important to turn off the power when they are not in use. The audio CODEC of the smart phone can provide some power management functions to switch the output of individual speakers, so as to avoid unnecessary power consumption. In addition, voltage regulators in system power management schemes typically do not provide the speaker with the power required to achieve maximum volume, so CODEC chip manufacturers use the in-chip speaker approach, which is to drive the speaker directly through the battery. Although this does not necessarily reduce power consumption, it also saves the need for additional voltage regulators.

ring

In recent years, cell phone ring tones have become more and more complex, from simple ring to polyphonic ringtones to various sounds that can be made into stereo WAV and MP3 formats. MIDI has become the standard format for polyphonic ringtones, and many manufacturers have introduced proprietary low-power MIDI chips for this application. To integrate the MIDI chip in the audio subsystem, additional analog inputs are required on the CODEC.

These additional inputs are also useful for the connection of FM radio ICs and provide additional functionality for multimedia applications. The generation of MIDI audio can of course also be generated by the audio CODEC, but the trend in the market today is stored in a special ringtone file and played through the existing Hi-Fi DAC. The CODEC chip manufacturer lacking the MIDI software library is not active. Go and do this.

Conclusion

What is the next step for a smart phone? As far as Hi-Fi stereo is concerned, it is already a must-have system function. As for the competition between I2S and AC97 on mobile audio systems, the competition will continue. Some people prefer a simpler I2S interface, but others prefer a low pin count and AC97 that can easily run at different sample rates. In the case of smart phones, most low-power processors currently support both specifications, and it seems that the two will coexist. However, for CODEC vendors, it is more difficult to support two specifications at the same time, because the VRA (variable rate audio) function of AC97 needs a different frequency architecture than I2S, and many additional digital circuits are needed to achieve .

However, will smartphones go from stereo to multi-channel surround sound format (Intel's Azalia) like the PC world? In the near future, this possibility is not seen, because today's multi-channel effect is dazzling, but the cost and power consumption of the chip are still too high, which is not acceptable to the mobile phone world. But today's negative answer, there are still big variables in the future electronic world, no one can be right.

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