bug: speech rate behavior on iOS

[dwassmer]

Plugin version:
4.1.0

Platform(s):
iOS

Current behavior:
Non linear behavior of speech rate on iOS.
Providing a range between 0.1-0.5 results in a slowly spoken text (half or smaller than default speech rate), where a value between 0.6 - 1.0 is always almost the same as the default speech rate. The jump in speed between 0.5 and 0.6 is considerably higher than between 0.6 to 0.7 for example.

With #36 speech rate is adjusted before assigning it to self.ttsUtterance?.rate. The following table shows (assuming AVSpeechUtteranceDefaultSpeechRate is 0.5) the plugin speed rate config compared to the utternace.rate

speed rate	utternace.rate
0.1	0.05
0.2	0.1
0.3	0.15
0.4	0.2
0.5	0.25
0.6	0.515
0.7	0.5175
0.8	0.52
0.9	0.5225
1.0	0.5

May I do not get the whole picture on this, but on devices I have tested (iOS 15.7.9, iPadOS 16.3, iPadOS 17.6.1) this makes it very hard to set an appropriate speed rate. What if I need the utternace.rate to be around 0.4? Also actual speed at 0.9 is even higher than with 1.0.

Expected behavior:
More linear behavior when using values between 0 and 1.

Steps to reproduce:

1. Pull https://github.com/robingenz/capacitor-plugin-demo/tree/main
2. Install and run on iOS
3. Announce text at different speed rates.

Related code:
#36

Other information:
Related bug: #34

Capacitor doctor:

💊   Capacitor Doctor  💊 

Latest Dependencies:

  @capacitor/cli: 6.1.2
  @capacitor/core: 6.1.2
  @capacitor/android: 6.1.2
  @capacitor/ios: 6.1.2

Installed Dependencies:

  @capacitor/android: 6.1.2
  @capacitor/cli: 6.1.2
  @capacitor/core: 6.1.2
  @capacitor/ios: 6.1.2

[success] Android looking great! 👌
[success] iOS looking great! 👌

[robingenz]

Yes, it's not perfect yet. Feel free to create a PR.