This is a continuation of the Signal Chain Basics article #73 (Signal Chain Basics #73: Audio metering, enjoying the VU) where we discussed the basics of audio metering systems. There are four main ways that manufacturers implement audio monitoring discretely, especially in systems that are low on DSP audio processing. Integration time and peak levels usually are handled by the external analog signal conditioning. However, in systems with enough digital signal processing (DSP), millions of instructions per second (MIPS), and memory, this can be handled on chip.
Low end analog — comparators
Figure 1 shows a simple system using comparators to do the level monitoring. The key advantages to using this method are cost and design simplicity.
In Figure 2 we used the LM3914/5/6 LED metering products, which essentially are precision-trimmed comparators with additional support circuitry, such as current sources. Large range LED meters can be built by daisy chaining these devices. The datasheets for these devices have great examples of front-end circuits that can be used for this, and for the comparator and MSP430 solutions.
Many “Pro Audio” solutions on the market use the LM391x devices, as they contain 90 percent of the circuitry to implement a high quality, reliable, simple-to-implement analog VU meter.
Low-end mixed signal microprocessor
In my most recent design, I used the ADC10 peripheral on an MSP430G2231. The 10-bit analog-to-digital converter (ADC) allows capture of up to 60 dB of dynamic range (1024 linear steps). As I won't be listening to the output, the “distortion” really isn't critical. Absolute accuracy isn't guaranteed, but as the comparator is based in software (for example, If ADC10MEM < 0x01FF LED1 ON), it can be easily trimmed.
Figure 3 shows the basic connection. The ADC is running as quickly as possible with a basic “IF-THEN” comparator software matrix being implemented. This design can be extended by running the microcontroller with a UART interface, and allowing threshold levels to be modified via a USB-UART cable, etc. Additionally, for those feeling brave, the load on the 3V3 rail can be steadied by using a constant current source, so that a fixed 20mA (or whatever LED current you want) always will be drawn from the power rail. This also maintains a constant current through the ground plane.
High-end mixed signal
If your system requires any kind of audio processing, even something as simple as an audio crossover, a mini-DSP device, such as PCM3070 is an ideal drop-in solution. The Purepath Studio tools offer the ability to drop in blocks of code graphically to implement level meters. It’s even possible to use multiple blocks in parallel to monitor multiple channels, or to monitor multiple frequency bands.
Moreover, the PCM3070 can monitor both analog and digital (I2S) sources. Features usually integrated into the analog front-end can be integrated into the mini-DSP process flow. Multiple LEDs can be driven either by GPIO directly on the MSP430, or by using an external shift register, such as the SN74AHC595.
Devices with programmable DSPs are wonderful. However, consider that a microcontroller (like an MSP430) typically is required to get data in and out of the device and to drive your LED displays. That inherently adds cost and a software overhead to the solution. Yet, such a solution is capable of doing much more in real time than simply processing the audio. The PCM3070 also can do the precision rectification required in software. That immediately removes the front-end circuitry required. No more buffer op amp, rectification diodes, and so on.
It has become very apparent over the audio editions of signal chain basics that a number of audio experts and “audiophiles” also read this column. I’d be really interested in understanding how you have implemented this kind of function in your products. Are there any downsides/gremlins in these implementations that you’ve experienced? Until then, keep your signals clean and noise free.
Please join us next month when we will address one of the most common problems that engineers have with INAs and PGAs: common-mode limitations.
For more information, visit www.ti.com/audio-ca.
— Dafydd Roche is an Audio Systems Engineer with TI’s High-Performance Analog team, and a graduate from the University of York (UK). When he isn’t busy pulling out his hair trying to find new ways to get an equal balance of performance, reliability, and cost, Dafydd pours his passion and knowledge of audio and music making into his work, helping designers and consumers get cleaner inputs and louder outputs. He may be reached at email@example.com