Systems and methods for autocorrelation based assessment of PPG signal quality

1. A method of assessing photoplethysmographic (PPG) signals generated based on transdermal optical data, the method comprising:

acquiring, by a computing device, a sequence of images representing transdermal optical data of a subject;

generating, by the computing device, a sequence of downsampled color frames corresponding to the sequence of images by downsampling a respective color frame for each image of the sequence of images;

generating, by the computing device, a PPG signal using color intensity values corresponding to central image blocks defined across the sequence of downsampled color frames, each color intensity value generated using a corresponding central image block defined in a corresponding downsampled color frame in the sequence of downsampled color frames;

computing, by the computing device, using a sliding window, a plurality of normalized autocorrelation signals of the PPG signal, each normalized autocorrelation signal corresponding to a portion of the PPG signal defined by a corresponding position of the sliding window;

determining, by the computing device, using the plurality of normalized autocorrelation signals, a plurality of quality metrics indicative of a quality of the PPG signal, determining the plurality of quality metrics includes determining, for each normalized autocorrelation signal, a respective peak; and

generating, by the computing device, based on the plurality of quality metrics, an indicator indicative of the quality of the PPG signal,

wherein determining the respective peak for each normalized autocorrelation signal includes:

penalizing correlation values of the normalized autocorrelation signal using powers of corresponding indices; and

identifying an index of the respective peak as an index of a maximum of penalized correlation values.

2. The method of claim 1 , further comprising:

comparing, by the computing device, for each normalized autocorrelation signal, the respective peak to a threshold value; and

determining, by the computing device, the quality of the PPG signal based on comparing the respective peak to the threshold value.

3. The method of claim 1 , further comprising:

determining, by the computing device, a local period of the PPG signal using respective peaks of the plurality of normalized autocorrelation signals.

4. The method of claim 2 , further comprising:

aborting, by the computing device, acquisition of the sequence of images if respective peaks of a subset of the plurality of normalized autocorrelation signals corresponding to a predefined cumulative time period are below the threshold value.

5. The method of claim 1 , further comprising at least one of:

displaying, by the computing device, a graph of at least a portion of the PPG signal;

displaying, by the computing device, a signal quality bar depicting the indicator indicative of the quality of the PPG signal; or

displaying, by the computing device, a timer indicative of time left for completing acquisition of the sequence of images.

6. A computing device comprising:

a processor; and

a memory storing computer code instructions, the computer code instructions when executed by the processor cause the computing device to:

acquire a sequence of images representing transdermal optical data of a subject;

generate a sequence of downsampled color frames corresponding to the sequence of images by downsampling a respective color frame for each image of the sequence of images;

generate a photoplethysmographic (PPG) signal using color intensity values corresponding to central image blocks defined across the sequence of downsampled color frames, each color intensity value generated using a corresponding central image block defined in a corresponding downsampled color frame in the sequence of downsampled color frames;

compute, using a sliding window, a plurality of normalized autocorrelation signals of the PPG signal, each normalized autocorrelation signal corresponding to a portion of the PPG signal defined by a corresponding position of the sliding window;

determine, using the plurality of normalized autocorrelation signals, a plurality of quality metrics indicative of a quality of the PPG signal, determining the plurality of quality metrics includes determining, for each normalized autocorrelation signal, a respective peak; and

generate, based on the plurality of quality metrics, an indicator of the quality of the PPG signal,

wherein in determining the respective peak for each normalized autocorrelation signal, the computing device is configured to:

penalize correlation values of the normalized autocorrelation signal using powers of corresponding indices; and

identify an index of the respective peak as an index of a maximum of penalized correlation values.

7. The computing device of claim 6 , further configured to:

compare, for each normalized autocorrelation signal, the respective peak to a threshold value; and

determine the quality of the PPG signal based on comparing the respective peak to the threshold value.

8. The computing device of claim 6 , further configured to:

determine a local period of the PPG signal using respective peaks of the plurality of normalized autocorrelation signals.

9. The computing device of claim 7 , wherein the processor is further configured to:

abort acquisition of the sequence of images if respective peaks of a subset of the plurality of normalized autocorrelation signals corresponding to a predefined cumulative time period are below the threshold value.

10. The computing device of claim 6 , wherein the processor is further configured to perform at least one of:

display a graph of at least a portion of the PPG signal;

display a signal quality bar depicting the indicator indicative of the quality of the PPG signal; or

display a timer indicative of time left for completing acquisition of the sequence of images.