Telephone CODEC usually has a pulse code modulation (Pulse Code ModulaTIon, PCM) interface. Strictly speaking, the concept of PCM includes most of the digital formats we are using today, including I2S; the original intention of PCM is to distinguish digital coding from analog technologies such as frequency modulation. However, in digital phones, PCM usually refers to a specific, single-tone data format that is not compatible with Hi-Fi stereo.
The emergence of computer audio also gave birth to the emergence of another type of interface. Since the quality requirements are similar to the existing consumer audio market, there is a need to play recorded audio files at different sampling rates (especially 8kHz, 44.1kHz and 48kHz). Although it is possible to convert the sample rate in software, it is also very expensive. Therefore, the AC 97 standard, which is currently widely used, delegates this task to the CODEC for processing, and very high efficiency can be achieved through specialized hardware. At present, AC 97 has become the leading industry standard in the field of computer audio.
The portable system initially maintained its original features: personalized CDs, mini discs and MP3 players used I2S DACs, mobile phones retained PCM technology, and PDAs with audio enhancement generally used the same AC 97 as desktop computers. CODEC. Therefore, this is simply shocking. The first-generation combined system usually contains two circuit boards, a telephone and a PDA. The two are arranged back-to-back in a chassis, and the PCM call CODEC is controlled by the communication processor. Hi-Fi stereo (AC 97 Or I2S) CODEC is controlled by the application processor. However, CODEC is not designed for such applications at all, with little or no consideration for the interconnection between the two audio subsystems. Therefore, engineers often insert discrete solid state switches in the analog signal path, but this also introduces pops, clicks, and harmonic distortion, and takes up PCB space.
Integration
Obviously, it is very popular to tailor an integrated solution for the above applications. Inspired by system-on-chip (SoC) design concepts, some manufacturers have integrated stereo DACs or CODECs into large-scale integrated circuits. However, this type of approach cannot achieve the audio quality achieved by the dedicated audio chip. Combining power management ICs and audio ICs often compromises audio quality because power conditioners often introduce noise into nearby audio signal paths.
Integrating audio into digital ICs is also very tricky, because true Hi-Fi components generally require the 0.35μm process most suitable for mixed-signal applications, and digital logic circuits have grown to 0.18μm or below. For these two single-chip integration methods, either the performance of the simulation category is compromised, or the size of the entire chip is increased to an unacceptable level (if the entire IC is designed according to a larger geometric principle).
Loudspeaker amplifiers generate a lot of heat, which requires proper heat dissipation devices, and is particularly difficult to integrate. Many modular chips lack this function, so they cannot be treated as a true "system-on-a-chip" solution. They usually require an external speaker driver IC. Another common problem is that the IC size is as small as possible, resulting in an insufficient number of analog inputs or outputs. In a square package (such as a QFN quad flat package, no leads), the pins are arranged around the IC, and the length of each side can be extended by 1mm to accommodate a few additional pins, resulting in a large increase in IC pitch. For example, increasing from 5 × 5mm to 6 × 6mm requires an additional 11mm2 PCB area; if starting from a 10 × 10mm package, the additional area is 21mm2.
The dedicated audio IC avoids these problems. By integrating other mixed-signal functions (such as touch screen digital processing) with call CODEC and Hi-Fi CODEC, the total number of chip pins can still be reduced. Here, the call CODEC is integrated in the phone chipset, Hi-Fi CODEC with additional analog input, output and internal mixing functions may be more appropriate. On the other hand, a dual CODEC with a dedicated PCM interface for Bluetooth connection is also very beneficial.
There are many ways to achieve audio integration. Sharing ADC and DAC can reduce the hardware cost, but cannot play or record two audio streams at the same time. Configuring a dedicated converter for each function can overcome this problem, and can also extend the life of the battery, because the power consumption of the telephone-grade (telephonygrade) audio module can be designed to be lower than the power consumption of the Hi-Fi function. However, this type of solution increases the cost of silicon wafers. A common compromise is to use some discrete DACs, but share these ADCs. This allows audio to be played at the same time as the call (such as the ringing or music of the second call), but the application processor will not record these during the call-this is an acceptable limitation because the user is It is unwilling to check how valuable the second call is when you use it. Turning off one channel and running the other channel at a low sampling rate can control the power consumption of the DAC.
Timing and interface
Although it is possible to share internal circuit modules between the two areas of communication and applications, this situation does not occur in interface applications. This is because each audio data stream runs on an independent clock domain at its own clock frequency. Only in this case, the combined smart phone CODEC needs a PCM interface and an independent I2S or AC 97 connection.
In fixed systems, the audio clock is usually generated by a quartz crystal oscillator. For example, AC 97 stipulates that a CODEC that complies with this specification should have an on-chip oscillator connected to an external 24.576MHz (512 × 48kHz) quartz crystal, while I2S parts use multiple sampling rates, most commonly 256. However, in the design of smart phones, driven by the additional power consumption, PCB space, and the cost of the clock quartz crystal, the designer had to derive the Hi-Fi clock from another clock already on the PCB. Although the complex odd frequency ratio (odd frequency ratios) needs to be implemented by a phase-locked loop (PLL), this solution still prefers to use an external quartz crystal because the low-power, low-noise PLL can be integrated into the mixed-signal IC at a lower cost in. This method is also suitable for some clock signals that other subsystems may need, such as the standard 27MHz clock of the MPEG decoder in video-enhanced telephony applications. In I2S CODEC, different sampling rates require different clock frequencies. As long as the word clock LRCLK (which is the sampling rate) is simply multiplied by 256 or any other fixed number, the PLL can provide the correct clock in each case. Therefore, component manufacturers generally tend to integrate one or two PLLs in their smart phone CODEC.
microphone
In the design of smart phones, many of the most difficult problems are related to the microphone. Usually we consider at least two types of microphones: built-in (internal) microphones and external microphones, the latter being part of the headset. In order to eliminate noise or achieve stereo recording, some auxiliary internal microphones may be needed, and the car hands-free device may be connected to another external microphone. In addition to calls, these microphones can also record call records under the control of the application processor, or even the sound track of a video clip.
In order to completely eliminate off-chip switching, the smartphone CODEC needs to provide sufficient microphone input, preferably with independently adjustable gain, and flexible internal routing paths to cover all use cases. In addition to the recording function, a "side tone" function should also be provided. This adds an attenuated version of the analog output to the analog output, and cell phone callers can hear their own voice. When plugging or unplugging the headset, plug-in detection enables seamless switching between the internal microphone and the external microphone.
Noise is another common concern. Interference signals are generated by high-frequency and digital parts in the line. After these signals are captured by the PCB wiring carrying the microphone signal, they will be amplified by the on-chip preamplifier. Careful circuit board wiring plays a big role in avoiding this problem, and using a differential microphone input is another effective method. However, the input has its own unique wiring requirements: the two PCB lines must be distributed side by side and adjacent to each other, so that any noise that appears on one line will also appear on the other line, making it Completely disappeared from the microphone preamplifier.
Noise elimination is a separation issue and requires two microphones: one is responsible for picking up speaker speech with background noise, and the other is only responsible for background noise. In the analog field, simple subtraction operations can hardly achieve satisfactory results, because the two noise signals differ in phase and amplitude, depending on the direction in which the noise comes in. Digital signal processing is also required here, but by digitizing these two microphone signals, the CODEC must make the task easier.
Another type of noise that occurs in outdoor applications is wind noise. This noise is mainly limited to frequencies below 200 Hz, and can be greatly reduced by a high-pass filter. The simplest solution is to use a coupling capacitor with a smaller capacitance at the input stage of the microphone. However, this also prevents the application of microphones in indoor music recording-this may make the bass disappear. For dual-use microphones, filtering should therefore be an alternative. It is worth mentioning that most audio ADCs already have a built-in high-pass filter to remove DC bias in digital signals. Integrated circuit manufacturers have customized this feature for mobile phone applications: a few Hz is used for Hi-Fi, and 100-200 Hz is used for calls with wind noise filtering enhancement. Of course, analog filtering and digital filtering can also be combined to create higher order filter characteristics.
Headphones and earplugs
Specific analog circuits are also required for mobile phone headsets. The first obvious task is to reroute the output signal from the earpiece or other speakers to the headset when the headset is plugged in. Although mechanical switches with integrated sockets can do this, they are bulky and costly.
Moreover, the signal level used for the speaker may not be suitable for headphones. Independent analog outputs for earpieces, speakers, and earbuds with independent volume control can solve this problem, and also allow the use of a relatively simple socket. Although a mechanical switch is still required, a single-hole, single-throw mechanical switch with one end connected to the ground pin is sufficient, so the socket only requires an external pin. However, in a multimedia phone, the activation of the switch does not necessarily represent the insertion of the headset; for a standard socket, this may be equivalent to a headset without a microphone. Therefore, the presence or absence of the microphone should be independently detected.
For electret microphones, this can be achieved by monitoring the microphone's bias current: if no current flows, there is no microphone plugged in. Conversely, an unusually large bias current also makes sense: To avoid adding an extra contact to the standard headphone / earphone jack, the button used to answer calls from earphones (also called hookswitches) usually shorts out the microphone . Therefore, the increase in bias current indicates that the hookswitch is depressed. By adding a current sensor to the on-chip microphone bias circuit, the smartphone CODEC can detect these two situations and automatically take the correct action for each situation.
horn
Recently, the number and output power of speakers in mobile phones have increased. However, in the 1990s, a single earpiece was already a basic configuration. The modern clamshell design features inside and outside speakers, which can play their respective sounds when the phone is turned on or off. Stereo ringing requires two external speakers, and the popular hands-free feature may require another "large" (refer to mobile phone standard) speaker in addition to a small handset. For microphones, providing a dedicated analog output for each speaker has many advantages over off-chip switching. Since horn amplifiers can absorb large supply currents, it is critical to cut off their power supply in a non-excited state. Smart phone CODEC provides more and more micro power management functions, allowing each output to be turned on or off, avoiding any unnecessary battery consumption.
In addition, the voltage regulators in existing power management schemes often cannot provide sufficient current to drive speakers operating at maximum volume. In response to this problem, CODEC manufacturers have designed some on-chip speaker amplifiers to operate directly from the battery (the typical voltage of a lithium-ion battery is around 4.2V) instead of the adjusted power supply voltage. Although this usually does not save energy (the horn amplifier requires additional energy, as does the voltage regulator), it does not require an additional voltage regulator.
Bell
Compared with the past few years, the complexity of the bell has been steadily increasing. It has evolved from the monotonous "beep" sequence tone to a multi-tone song, even WAV and MP3 segments, literally any type of sound Can be made into stereo. MIDI has become a multi-tone bell interface standard, and many manufacturers have developed low-cost MIDI chips for this new application. Integrating such ICs into the audio subsystem requires CODEC to have an additional analog input.
These additional inputs are also useful for connecting FM radio ICs, which add another function to multimedia devices. The MIDI ringtone generating circuit should of course be integrated in the CODEC; however, the general trend is to save the random ringing as a sound file and then play it through the existing Hi-Fi DAC, which limits the appeal of this idea to IC manufacturers Force, because their IP products have not yet included MIDI.
future development
So, what is the future of smartphone audio? At present, some digital audio trends that deserve attention include: the shift from stereo to multi-channel surround sound format. The recently launched "Azalia High Definition Audio" standard is likely to be adopted by most PCs and Used in laptops.
Although not long ago, those who once laughed at the idea of ​​placing stereo speakers on mobile phones have been proven wrong by today's market reality; but in the future that can be met, handheld devices cannot be developed into multi-channels. Similarly, the cost and power consumption of Azalia ’s new features cannot be proven higher than AC 97. The controversy between I2S and AC 97 is continuing. Some designers prefer the less complex I2S interface, while others prefer the smaller pin count of AC 97 and its various sampling rates that are easy to handle. Since many low-power processors used in mobile phones are now able to provide dual-standard audio interfaces that can satisfy the preferences of the above two camps, these two standards may continue to coexist. On the contrary, designing a CODEC that supports two standards is quite difficult, because VRA (variable data rate audio) represented by AC 97 requires a completely different timing scheme from I2S, and a significant number of additional digital circuits.
The experience of using an audio clock to successfully integrate the application processor and communication processor into a single digital device will make it possible to merge the call interface and the Hi-Fi audio interface, and may eventually prompt the industry to return to the low-complexity CODEC. But for now, IC manufacturers are focusing on integrating other existing mixed-signal components into their audio CODECs, including touch screen functions, voltage regulators, and power management.
Until now, the process of integrating imaging solutions (ready availability) has prevented the integration of audio with camera or video functions, but this is by no means static (a law set in stone). At the same time, audio features such as 3D enhancement, graphics compensation, and dynamic compression are beginning to spread, and sound quality, power consumption, and package size are becoming more sophisticated.
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