Cadence and media content phase alignment

1. A media-playback device for aligning play back of media content to foot strikes for a user performing a running activity, the media-playback device comprising:

a content output device that operates to output media content;

a cadence-acquiring device comprising a movement-determining device and a cadence-determination engine configured to determine a cadence based on movement data captured by the movement-determining device, wherein the movement-determining device comprises three orthogonally-oriented accelerometers and the cadence-determination engine is configured to:

capture sequences of measurements from each of the three accelerometers;

filter the sequences of measurements based on frequency to generate filtered sequences;

identify a sequence of measurements of the filtered sequences corresponding to the cadence based on calculating energy values for each of the filtered sequences;

a phase-delay calibration engine configured to determine phase delay values for at least one cadence value, wherein the phase-delay calibration engine determines the phase delay values based at least in part on a time required to detect a foot strike within the running activity;

a cadence-based media content selection engine configured to identify a media content item based on the cadence determined by the cadence-acquiring device, wherein the media content item comprises music with a tempo that corresponds to the cadence; and

a phase-aligned media playback engine configured to:

identify a wave action position corresponding to a particular foot strike within the running activity, wherein the phase-aligned media playback engine identifies the wave action position by being programmed to:

calculate a positive energy value of at least one of the sequences of measurements, wherein the positive energy value corresponds to energy of a portion of the at least one of the sequences of measurements that is greater than zero;

calculate a negative energy value of at least one of the sequences of measurements, wherein the negative energy value corresponds to energy of a portion of the at least one of the sequences of measurements that is less than zero;

identify the wave action position as a falling zero crossing when a magnitude of the positive energy value is greater than or equal to a magnitude of the negative energy value; and

identify the wave action position as a rising zero crossing when the magnitude of the positive energy value is less than the magnitude of the negative energy value;

align the identified media content item to the running activity using at least one of the determined phase-delay values; and

cause the content output device to output the aligned media content item.

2. The media-playback device of claim 1 , wherein the phase-aligned media playback engine is further configured to:

identify foot strikes within the running activity based on the movement data; and

cause the media-output device to output the identified media content item so that a beat of the media content item is output coincidentally with an expected foot strike.

3. The media-playback device of claim 1 , wherein the phase-aligned media playback engine is further configured to:

identify a wave action position within an oscillation of a filtered sequence of measurements captured by the measurement-determining device corresponding to the foot strike based on calculating an orientation of the media-playback device.

4. The media-playback device of claim 1 , wherein the phase-aligned media playback engine is further configured to:

determine a phase delay value corresponding to the determined cadence; and

align the identified media content item to the running activity based on the determined phase delay.

5. The media-playback device of claim 1 , wherein the cadence-based media content selection engine is configured to identify a media content item that includes music with a tempo that is within a predetermined threshold of the cadence.

6. The media-playback device of claim 1 , wherein the cadence-based media content selection engine is further configured to:

monitor for changes in cadence; and

upon detecting a change in cadence:

identify a second media content item based on a changed cadence;

align the second media content item to the running activity; and

cause the media-output device to playback the aligned second media content item.

7. The media-playback device of claim 1 , wherein the media playback engine is configured to identify wave action positions corresponding to foot strikes.

8. The media-playback device of claim 1 , wherein measurements are captured at a sample rate in the range of 20-200 Hz.

9. The media-playback device of claim 1 , wherein the media content item is stored locally on the media-playback device.

10. The media-playback device of claim 1 , wherein the media content item is streamed to the media-playback device by a media-delivery system based on a request for media content items that specifies the cadence.

11. The media-playback device of claim 1 , wherein the media-playback device is a smartphone.

12. A method of cadence-based media playback for use during running activities comprising:

determining a cadence associated with a running activity based on acceleration data captured by a plurality of accelerometers, wherein the acceleration data comprises sequences of acceleration sample data captured from each of the plurality of accelerometers over a duration of time;

identifying a media content item based on the determined cadence;

identifying a wave action position corresponding to a foot strike within the running activity by:

calculating a positive energy value of at least one of the sequences of acceleration sample data, wherein the positive energy value corresponds to energy of a portion of the at least one of the sequences of acceleration sample data that is greater than zero;

calculate a negative energy value of at least one of the sequences of acceleration sample data, wherein the negative energy value corresponds to energy of a portion of the at least one of the sequences of acceleration sample data that is less than zero;

identify the wave action position as a falling zero crossing when a magnitude of the positive energy value is greater than or equal to a magnitude of the negative energy value; and

identify the wave action position as a rising zero crossing when the magnitude of the positive energy value is less than the magnitude of the negative energy value:

phase aligning the identified media content item to the running activity using a phase delay value and the identified wave action position, wherein the phase delay value is determined based at least in part on a time required to detect a foot strike within the running activity; and

playing back the aligned media content item.

13. The method of claim 12 further comprising:

filtering the sequences of measurements based on frequency to generate filtered sequences;

identifying a cadence signal corresponding to the cadence of the running activity from the filtered sequences; and

analyzing the cadence signal to identify a wave action position corresponding to a foot strike within the running activity.

14. The method of claim 13 , wherein analyzing the cadence signal comprises:

identifying a portion of the cadence signal corresponding to an oscillation of the running activity;

determining an orientation of the cadence signal relative to the ground; and

selecting a point within the oscillation that corresponds to an acceleration towards the ground stopping as the wave action position.

15. The method of claim 12 , further comprising:

determining a phase delay value corresponding to the determined cadence; and

aligning the identified media content item to the running activity using the determined phase delay.

16. A method of calibrating a cadence-based media playback device for use during running activities comprising:

generating media output, wherein the media output has a tempo;

determining a cadence associated with a running activity based on acceleration data captured by a plurality of accelerometers, wherein the acceleration data comprises sequences of acceleration sample data captured from each of the plurality of accelerometers over a duration of time;

determining whether the tempo of the media output corresponds to the determined cadence; and

upon determining that the tempo of the media output corresponds to the determined cadence;

identifying a wave action position corresponding to a foot strike within the running activity by:

calculating a positive energy value of at least one of the sequences of acceleration sample data, wherein the positive energy value corresponds to energy of a portion of the at least one of the sequences of acceleration sample data that is greater than zero;

calculate a negative energy value of at least one of the sequences of acceleration sample data, wherein the negative energy value corresponds to energy of a portion of the at least one of the sequences of acceleration sample data that is less than zero;

identify the wave action position as a falling zero crossing when a magnitude of the positive energy value is greater than or equal to a magnitude of the negative energy value; and

identify the wave action position as a rising zero crossing when the magnitude of the positive energy value is less than the magnitude of the negative energy value; and

calculating a phase delay value for the determined cadence based on the identified wave action position and a beat within the media output, wherein the phase delay value is determined based at least in part on a time required to detect a foot strike within the running activity

storing the phase delay value for use in phase-aligning media content to a running activity for play back of an aligned media content item during the running activity.

17. The method of claim 16 , wherein the tempo of the media output is selected to correspond to a goal cadence selected from a predefined list of cadence values.

18. The method of claim 16 , wherein the tempo of the media output is selected based on the determined cadence.