Signal Amplification on Smartphone´s Audio — Part II
Some time ago, we released a post on which we were giving a basic introduction about the role of the signal amplification on the smartphone´s sound and some basic concepts. It is recommendable to have read that part before going ahead, you can find the first part on this link
Today´s post is about the three most common Amplifying Structures used in smartphone´s industry. These ICs are based on a power amplifier (PA) adding some other components which provide to the system a certain degree of intelligence in order to optimize the amplification process.
The PA used as the amplifying unit is classified according to its properties which vary a lot depending on its electrical design. In our case, in most of the Amplifying Structures, the PA unit used is class-D as a consequence of the good balance this class has in terms of efficiency and performance.
Naked PA
These chips just contain the PA. Usually, the gain of the PA can be adjusted by HW through some external resistors and low&high cutoff frequencies through basic RC passive filters at the chip signal input.
Gain setting adjusts the output power level. This adjustment must take into account the worst use case so that the speaker is never overloaded (a) or the output on the PA is never saturated (b).
a)You need to consider the output level for the greatest possible input signal to make sure it won´t exceed the speaker max. rated power (if that happens you may get high distortion or even damage your speaker).
b)You need to consider the lowest possible battery level on the PA supply power, to make sure the output signal will never reach it (if that happens the output signal will be saturated).
*Typical designs using the “Naked PA” use an external DC-DC regulator to keep the power supply constant at 5V and avoid b).
This solution totally lacks of awareness about system status or input signal and that makes it low efficient in terms of how capable it is of keeping the system boosted to maximum whichever the use case/system status is.
This type of amplifying structure is used in cost-effective devices. They are simple, cheap and easy to integrate into any hardware solution.
AGC/DRC (Automated Gain Control/Dynamic Range Control)
This Amplifying Structure´s name is quite clarifying. Automated Gain Control sounds like a system which is somehow capable to automatically adjust its gain and this is one of the lacks that Naked PA has.
We explained before that fixing the gain requested to consider the worse use case (greater input signal) in order not to overload the speaker/saturate the output signal. What would happen if we were capable of using a variable gain which could be adjusted depending on the input signal level? That is exactly what the Automated Gain Control does.
The main advantage of that is that you can use higher gains for low input level signals. This way, we can get greater output signal for all those cases on which the input level is lower than the worse case.
Why do we call this system Dynamic Range Control? DRC, as well known simply as compressor, is an audio processing block which compresses the input dynamic range into a smaller output dynamic range.
Dynamic Range is the amount of different possible levels which can be observed. As you can see in the picture below, the main side-effect for using the compressor is that you will loose some dynamic range as a trade-off for the loudness increasement.
In order to give a better understanding about the difference between the Naked PA and the AGC, we can make an example. Using a piece of song which shows a great dynamic (the difference between lowest and highest level), we have simulated the effect of a 5dB Naked PA and an AGC which reaches same maximum output level (assuming both of them would be loading the same speaker).
You can clearly observe in that example, that the part of the song with the highest level is amplified quite the same, but the initial part, with low level, is boosted much more on the AGC, as a consequence of its adaptive gain which can be much higher for that part of the song.
Additionally, the AGC/DRC includes in most of the cases the DC-DC converter which sets the power supply to max voltage according to the maximum output level expected to avoid signal saturation.
These amplifiers are used nowadays in entry-level smartphones as they are still quite inexpensive and can enhance the loudness. The main advantages of this platform compared to Naked PA are basically thanks to the system awareness of the input level signal before applying a certain amplification gain.
Smart PA
There is a big loudness improvement on AGC related to Naked PAs but the Amplifying Structures “changed the rules” for reaching an outstanding improvement never seen before.
Up to the moment, we are always trying to maximize the amount of power delivered to the speaker for not overloading the speaker, understanding that the limit of the speaker is always the rated power specified by the speaker supplier. Is it the real limit? can we go beyond that limit?
If we just think on these questions, we can clearly understand, that if we could be able to find a new speaker limit which would allow delivering more power to it, the overall performance (PA+Speaker) could be improved.
- How do suppliers determine the rated power for a speaker?
The rated power is defined by analyzing the reliability of the component while playing a wideband signal (20Hz to 20kHz) at a given power for a long time (96 hours). If the speaker performance remains the same after several tests, we can assure that the speaker can handle that rated power.
This wideband signal is usually pink noise. This signal has a particular frequency density and peak to RMS relation (also known as crest factor) which mimics the average audio content which can be played through the speaker.
What if I am not playing all the time audio content similar to that pink noise? What if we can analyze in real time the type of audio content being played and adapt my system to maximize the signal delivered to the speaker?
- Which are truly the key parameters which set the speaker´s limit?
We need to understand what exactly is relevant to the speaker´s reliability and analyze those parameters in real-time based on the content being played.
Well, signal power looks like relevant, but it is not the key parameter to look into to check if the speaker may be or not reaching its limit. Speaker overload point can be tracked by two very low-level physical magnitudes: Temperature and Distance.
The speaker limit is truly fixed by the maximum temperature its coil can handle, and the maximum excursion (elongation) of its membrane. If we are able to observe and control the coil´s temperature and the membrane´s excursion, we can deliver as much power as we want.
The rule has been changed, speaker limit is not the power anymore but its excursion and temperature.
This new approach will request from the Amplification System the capability of tracking the status of the speaker. SmartPAs will be aware not only of the audio input signal but also of the speaker status.
- Do these systems require a greater power supply for avoiding saturation?
They do! As we are claiming, these systems are capable of delivering much more power to the speaker without reaching its limit (excursion/temperature ) so you may think that we should also increase the power supply voltage to avoid saturation, and that is correct.
While typical AGCs use a 5V power supply, it is normal that you can use a 10V power supply to avoid saturation under certain power peaks demand. Keep in mind making double the voltage, makes four times bigger the power demand!
This Amplifying Structures also use its smartness in terms of power handling! Any power supply has a constant current drain known as the quiescent current which is proportional to its output voltage. That means, the greater the voltage the power supply provides, the greater the standby consumption will be. Smart PAs, are aware of the input signal level so they can predict their output level. That level will not always reach the maximum output voltage so, why do we need to keep always the power supply set to max voltage? We don´t have to! Smart PAs use Smart Power Supplies, which dynamically adjust the power supply voltage to the minimum necessary level to avoid saturation.
We are not even close to fully explain how SmartPAs work, but we have just given some insights into the trigger which enabled such a big improvement on the Amplifying Structures.
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I hope you all have enjoyed this article and keep reading. We will release soon the third and last part of this post on which we will go deep into Smart PA operation and explain in detail how these amazing amplifiers work.
