What is Hybrid Gain Control and How Does it Improve Audio Quality?

A Q&A with Cirrus Logic Product Line Manager, Jonathan Taylor.

Learn more: No Zipper, No Pop! How ADC Hybrid Gain Control Can Enable Smooth, Pop-Free Microphone Preamp Gain Changes

Creating a smooth gain control for digitally controlled microphone preamps is a longstanding challenge for designers of pro-audio equipment, particularly because it can result in audible pops or zipper noises when integrating control into the ADC.

So, how can you overcome these issues? To learn how, we spoke to our Product Line Manager, Jonathan Taylor, who explained how hybrid gain control is providing the answer.

What is hybrid gain control, and why is it important for audio processing?

Before you hit an ADC in audio recording, you have a pre-conditioning circuit that sets everything at the right levels and filters out non-audio noise. One important part of this chain is setting the volume on the incoming signal, particularly when you are recording via a microphone.

In this situation, you can control the gain (volume) on the input, which is a combination of digital gain and analog gain. These need to be controlled perfectly so they do not impact the end recording. For example, if the gain changes a significant amount at the wrong time in an audio recording, it will cause an audible “pop.”

The challenge that many people have today is that gain control is a convoluted process. You have digital gain on the main processor (MCU/FPGA) and an external analog gain, which together require a lot of tuning to work properly.

Instead, hybrid gain control pulls this all into one place. Our new ADCs/CODEC let users control both the external analog gain and the digital gain in one device. We also control the timing and size of each gain change, significantly reducing any pop noise. We take this complicated problem out of the user’s hands and improve the audio quality by reducing audible artifacts along the way.

How does hybrid gain control work at a technical level to reduce noise artifacts?

The technical background is that analog gain steps tend to be larger (1dB and up) and can cause a pop, if the gain change is not controlled correctly. In contrast, digital gain steps can be small and do not cause a pop.

Our technology controls the gain steps using a zero-cross that detects when the gain should be changed. When changing the analog gain, we can provide a perfectly timed digital gain step in the opposite direction, giving you a step of zero. We then slowly ramp the digital gain to the desired level without giving any additional pops.

In addition, changing the analog gain can sometimes create charge injection (resulting in another pop) – we can block this through transient masking.

How does it compare with traditional approaches in terms of audio quality and reliability?

Addressing these issues with traditional technologies is challenging. Customers have to build the entire system themselves, including a zero cross. They then have to manage the analog gain and the digital gain separately, before tuning everything in so there aren’t any pops. This requires a high level of technical knowledge and the right pieces of equipment.

What is innovative about the hybrid gain approach compared to other technologies?

What is special about the hybrid gain approach is that it significantly simplifies the pro-audio equipment design process by bringing all these processes into one place where we integrate the key components. That’s a crucial step forward for customers who need to maximize audio quality and want a way to achieve that more efficiently.

We are excited to see what customers do with the technology. Looking ahead, the system is very flexible, and there are some spare registers to add more features. That’s something we will talk more about down the line.

Learn more about Cirrus Logic’s new series of flagship audio converter products.

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