Dictation got worse? Check the microphone first

This week one of us noticed that dictation was making many more mistakes and the level bar on the panel looked lazy. We went through our recent changes looking for the cause. The cause was not in the code at all.

What we found

The Mac’s default input had stayed on a Bluetooth headset, with the input volume at 27%. We measured the loudness of the recorded phrases. Speech came in at about minus 48 decibels. On a normal microphone the same measure sits between minus 20 and minus 35. That is a very quiet recording, and the recognizer has little to work with.

It took a minute to see this in the recordings. Going through the changes would have taken an hour.

A second thing showed up the same day. After pressing the dictation key, the panel appeared, then there was silence, and only then the start sound. Our log said the app began recording 0.1 seconds after the key press, so the app was not slow. The headset was. When a Bluetooth headset opens its microphone, it switches from the music mode to the headset mode, and that takes a moment. The headset mode also has lower sound quality than music playback.

What the app does now

Each dictation now measures how loud the first second and a half of your speech is. If it is too quiet, the panel says so for six seconds: which microphone is listening and what its input volume is. Recording goes on, we do not stop you.

We chose the threshold on about fifteen hundred of our own recorded pieces. The warning catches about three quarters of the quiet hour we had, and fires by mistake on under one percent of normal speech. It is a rule of thumb, not a guarantee.

What you can check yourself

If dictation suddenly gets worse, look at the microphone before anything else.

• Open System Settings, then Sound, then Input. Speak and watch the level meter. If it barely moves, raise the input volume.
• Look at which device is selected. After you connect a headset, macOS may switch the input to it without asking.
• If you use a Bluetooth headset, expect a short pause when the microphone opens and a lower sound quality while it is on. Dictating with the Mac’s built-in microphone avoids both.

More hints are on the troubleshooting page.

Today: the app also lets you pick a separate microphone for dictation in Settings.