Today's converged portable multimedia devices have more and morefeatures integrated into their smaller systems. Audio is a basicfeature of any system marketed with multimedia capability.
System designers, however, often put more engineering focus onportable multimedia devices' “glamorous” features such as wirelessconnectivity, video processing, image capture and display. Hence, audiocircuits end up wedged in the system wherever space can be found amongthe “important” components, resulting in mediocre or downright pooraudio quality.
However, with a little care and attention, good audio quality can beseamlessly integrated with the myriad of other features demanded byconsumers.
Many sources of poor audio quality
There are many sources of poor audio quality in portable audio systems,but this article focuses on sources of audible noiseon analog audio signals.
Non-harmonically related noise – whether white (flat) or tonal -can be very annoying to the end-user. White noise is perceived as abackground hiss, which is very noticeable during quiet audio passages.
Tonal noise can be perceived as a buzz, hum or whine, depending onthe frequency content. Unnecessary noise corruption of audio signalscan be avoided with good system design PCB layout practices.
|Figure1: In Case A, the signal is amplified near the microphone before thetrace travels across the board and noise is coupled, resulting in asystem signal to noise radio (SNR) of 60dB. In Case B, the signal isamplified after the trace travels across the board and noise iscoupled, resulting in a system SNR of only 28dB|
Most portable audio systems use a DAC or codec IC to convert digital audio to analog signals.The layout surrounding the audio codec or DAC is critical. Codec or DACdevices have both analog and digital circuits in the same IC.
Thus, multiple supply pins are used for analog and digital power,often marked AVDD and DVDD. Other analog supply pin names can be HPVDD,DRVDD, SPKVDD and PVDD. Other digital supplies can be IOVDD and BVDD.
These power-supply pins are separate because the digital circuitscan be very noisy due to high-speed switching currents, and the analogcircuits are very sensitive to noise on the supplies.
An important point of audio system design and layout is that analogsupply pins should be provided with a very “clean” supply with minimumripple and transients. Any noise on the analog supply pins can corruptthe audio I/O signals in various ways.
In a portable audio system, the primary power source is usually abattery. The battery can be very noisy due to transients caused byother parts of the system, including wireless transceivers, storagedevices and displays.
Instead of using the battery voltage directly, a good practice is touse a low dropout (LDO) regulator withgood power-supply rejection ratio and low output noise. This wouldprovide the analog supply voltage to the audio codec or DAC, along withany other audio signal-path devices such as ampli- fiers, thus ensuringthat a “clean” supply is present for the analog circuitry.
Care should be taken to select an LDO regulator that has an adequatecurrent rating for the circuitry being powered as well. Properdecoupling capacitor usage on analog supplies is also important.
Large decoupling capacitors (10mocrofarad or greater) are useful for filtering the supply voltage.Smaller-value decoupling capacitors (1microfarad or smaller) are alsoneeded to supply fast transient currents when the IC calls for it.
Decoupling capacitors should be placed as close to the analog supplypins as possible, avoiding PCB vias between the capacitor, and thesupply and ground connections if possible.
Smaller decoupling capacitors should be placed closer to the IC pinsthan larger capacitors, as series resistance affects response times ofsmaller capacitors more noticeably.
Digital power supplies in audio converter ICs are much lesssensitive to noise than analog supplies, so the digital circuits can besupplied with a more efficient switched-mode power supply (SMPS).
SMPS usually has a higher output riddle and noise, but its 80percent efficiency and higher yield significantly improve battery life.Often, large decoupling capacitors are not necessary in digital powersupplies.
However, multiple smaller capacitors (i.e. 1 microFarad and 1nF)should be used to supply very high-frequency switching currents in thedigital circuitry. Again, smaller-valued decoupling capacitors shouldbe placed closer to the IC pins than larger capacitors.
Good design practices
Another source of noise corruption in portable audio systems is noisecoupling into analog I/O signals. Noise-coupling mechanisms can beinductive or capacitive, but good system design and PCB layout canminimize noise coupling.
One practice that yields very good noise immunity is usingdifferential signals wherever possible in the analog audio signal path.PCB traces used for differential signals should be routed as parallelpairs with matched impedance, so that any noise will couple equally (asa “common-mode” signal) into both sides of the differential signalpath.
The common-mode rejection property of the differential circuits usedrejects any coupled noise, which in turn, reduces the noise heard bythe user. Although there are cases where differential signals cannot beused, they are still a very useful tool.
Another good system-design practice is to use the highest possiblevoltage levels for signals that travel across the PCB and are thussusceptible to noise coupling.
It is valid to assume that the magnitude of coupled noise does notincrease with the level of the signal being transmitted. So if noiselevel stays constant and signal level increases, the SNR will increase.A higher SNR measurement indicates a higher performance audio system.
If a low-level signal is run across a PCB, gain must be applied.This can increase the noise along with the signal, reducing overallsystem SNR. Amplifying a lowlevel signal near its source is a goodpractice.
Figure 1 above shows anexample of this principle. A microphone generates a 25mVp-p signalA(t), which must go across a PCB and be amplified to 1Vp-p for furtherprocessing. The red box indicates the trace traveling across the board,which receives coupled noise, indicated by the signal E(t).
In Case A, the signal is amplified near the microphone before thetrace travels across the board and noise is coupled. This results in asystem SNR of 60dB. In Case B, the signal is amplified after the tracetravels across the board and noise is coupled. This results in a systemSNR of only 28dB and illustrates the difference in performance realizedby good system design.
|Figure2: Filtering the bias voltage with resistors and capacitors near themicrophone is a good practice.|
For signals that cannot be amplified near the source due to systemcost or size constraints, reducing the PCB trace length to the minimumpossible is critical. Short PCB traces are less susceptible to noisecoupling by capacitive and inductive mechanisms.
A final type of signal that should be carefully designed in systemswith built-in microphones is the microphone bias circuitry. Most of theelectret capsulemicrophones (ECM) used in portable audio systems require abias voltage in the 2-3V range. Often, the bias voltage is provided byan IC located away from the microphone.
<>In this case, the bias voltage picks up noise on its way to themicrophone capsule. This noise couples directly into the microphoneoutput. In this face, filtering the bias voltage with resistors andcapacitors near the microphone is a good practice. Figure 2 above shows a microphonecircuit design with a pseudodifferential connection and RC filter toattenuate noise from the bias voltage.
|Table1: Good quality audio can be achieved in a low-cost, low-power portableaudio system.|
All audio systems need some type of transducer so that users canhear the audio produced. In most systems, a headphone output ispresent. Some systems include a built-in speaker or outputs to driveexternal speakers.
Because headphones (>16 ohm) and speakers (> 4 ohm ) canrequire highpower signals, minimizing the impedance of the circuittraces associated with these transducers is critical. If PCB traceshave unnecessary high resistance, power can be lost in the PCB tracesand not delivered to the transducer.
This results in loss of audio quality, reduced battery life andunnecessary heat build-up in the system. Making speaker and headphonetraces as wide and as short as possible will reduce the impedance andminimize the negative effects mentioned.
Table 1, above summarizesall the recommendations discussed. When these practices are followed,good quality audio can be achieved in a low-cost, lowpower portableaudio system.