As you only want HR measurement, not pulse or ECG waveform, you are lucky.
Every method has its problems, and advantages. The problem with earclips in
sports is movement artifact. Very hard to deal with.  You can try to tape to
the skin a flat probe. They are made for pulse oximetry.
Time ago (in the pre-micro era) I was involved with a neonatal hr monitor,
and those problems were common. I had an advantage: newborn skin is almost
transparent, and does not change much from one to another person.
I agree with Wouter, the closed loop (analog) is my bet. And a lot of
filtering, and real time microcontroller supervision and adjustment.
If you only want the hr number, why not try a method similar to oximetry,
with two wavelenghts, and obtaining the ratio of signals? You can correlate
the oximetry wave with the pulse quite well, and than can allow to factor
out a lot of noise.

By the way, I read something later about a military sensor for NBC suits
using a contact microphone in the chest that allows to monitor HR,
respiration, speak and some other parameters via software filtering the
audio signal.

Hope this is useful,

Rafael

----- Original Message -----
From: Russell McMahon <apptech@PARADISE.NET.NZ>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Wednesday, November 19, 2003 6:44 AM
Subject: [EE:] Hear rate measurement - ideas sought


> I wish to measure human pulse rate in an exercise environment.
>
> Known options include (but are not limited to) ecg direct contact
> measurements using various sensor locations (chest (eg polar), hand to
hand
> etc) and blood flow variation (typically thumb or ear clip.)
>
> I want to try to produce good results using an ear clip sensor but may in
> due course end up using some other method.
>
> As ear clip sensors (IR LED to photo-detector via ear lobe & blood) are
> common enough one might expect that effective circuitry would be easy
enough
> to develop. Getting reliable signals is more annoying than I expected :-).
> Interestingly (or annoyingly) the sorts of problems that I experience
match
> those reported by users when I search the web. So maybe nobody ever really
> sorted this out well. A major factor is the wide variation in optical
> transmission through the ear lobe,  time to time, and user to user, and in
> session. Sensor movements or variations in clip tension on earlobe lead to
> shifts in the DC operating point as effective ear transmissivity varies
> which is seen as a substantial signal level change. As the AC levels are
low
> (millivolts) such changes swamp the detector circuit until the DC level
> again stabilises. Driving the detector via a feedback loop to keep its
> operating point stable helps but is not a perfect solution. The received
> signal must be reasonably aggressively low pass filtered to remove 50 (or
> 60) Hz hum artefacts. The low frequency of the desired signal leads to
large
> capacitor and resistor values (around the 1 uF and 1 to 10 Mohm range)
which
> makes DC operating point susceptible to capacitor leakage (so
electrolytics
> no good) and offset bias currents.
>
> I have a few more ideas to try (eg feedback modulate the IR LED rather
than
> changing the detector operating point) but I think it's liable to prove an
> annoying application still.
>
> Interestingly, I was unable to find a single circuit diagram on web
although
> there are many mentions of equipment using ear clips.
> Did turn up the complete circuitry for a pulse oximeter along the way :-)
>
> SO - anybody tried this or can suggest any useful circuit ideas?
>
>
>
>         Russell McMahon
>
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