Reduce click and pop noises in set-top boxes

Introduction
If you've ever turned your set-top box (STB) on or off while the television is on, and heard an annoying pop or other strange noise from your TV speakers, you may have wondered about the source of this noise and how it can be eliminated. Depending on the TV volume setting, such noises can be very disturbing. Because customer expectation is changing and the desire for better performance is increasing, STB engineers are always looking for ways to minimize these noises.

Noises are produced by voltage shifts on the STB's audio-output lines as the power state of the STB changes. Clicks and pops usually consist of unwanted transient and audible signals reproduced at the speaker. The audio amplifier's architecture, the AC coupling of the audio lines, and the ramp characteristic of DC supply voltages in the STB all contribute to the magnitude of this unwanted effect.

You can never completely eliminate these noises, but ICs that include Direct Drive Technology, such as offered by Maxim, can substantially reduce them in a STB. Engineers in the past have subjectively measured the transient noises on STB audio-output lines, but there is a way to take a more objective and universal approach to the phenomenon.

We first discuss a major source of this noise, and then explain how it was eliminated. Using the right approach and the right set of tools, we then illustrate the objective testing approach by measuring two different audio amplifier chips on a SCART STB.

Sources of pop and click in a STB
A major contributor of this phenomenon is the AC coupling capacitors placed on the STB output lines. Most audio amplifiers and codecs in a STB operate on a single supply, so the audio signal must be biased near the midpoint between supply rail and ground. (Otherwise, the amplifier's internal circuitry may not work correctly.) As a result, the STB output is a DC-biased audio signal.

Because the load (the TV) is referenced to system ground, there appears a voltage difference between load and amplifier that allows an unwanted DC current path. An AC-coupling capacitor is therefore introduced to isolate the load from the output amplifier and block the DC current. Another, more historical advantage of this capacitor was protection of the audio amplifier from external short circuits. (Audio jacks are exposed externally and connected by users, who may inadvertently short the outputs.) AC coupling capacitors provide good isolation and protection for the STB, but in modern amplifiers the circuitry for short-circuit current limiting largely negates the need of AC-coupling capacitors for that purpose.

When the audio amplifier is turned on (or off), its output ramps up (or down) to the bias voltage (or ground). The rate at which the coupling capacitor charges and discharges depends on its value², and ultimately determines the frequency of the transient noise. You might think you can increase the rate of charge by choosing an appropriate capacitor value, and thereby move the unwanted noises above the audio band³. Further investigation, however, shows that this approach is not so simple.

First, the initial ramping of output to its bias level depends primarily on the amount of current that can be sourced by the output stage of the amplifier. Capacitor size, however, only modifies the rate of output change (between bias voltage and ground) as the power ramps up or down. This is better explained by looking at Figure1(a) or Figure 2(a) (below). The sharper edge of the transient noise is due to power-up of the amplifier, and the slower edge, which brings the transient effect back to ground, is affected by the capacitor value.

Another factor that limits the size of the AC coupling capacitor is its filtering configuration. The capacitor and load form a highpass filter whose passband must be considered carefully to ensure that the audio band is covered and that no audio signal is degraded. A typical TV load is 10kΩ, but some STB audio amplifiers are designed for loads as large as 3.3kΩ (three 10kΩ loads in parallel). The most common value of AC-coupling capacitor for STB outputs is 10µF⁴. For that value, it is probably safe to say that none of the audio signal will be blocked or degraded by the filter.

An example of one approach to address the noise in audio/video routing in a STB are amplifier-filter ICs that operate on a single supply and incorporate Maxim's Direct Drive technology, which can guarantee an output DC offset that is within ±3mV of ground. That capability eliminates the need for AC-coupling capacitors on the output lines. Internal current-limiting circuitry also protects the integrated amplifier against accidental short circuits.