I think it is a saxophone. Probably alto.

how about a heckelphone?

It is definitely, 100 percent an english horn, aka cor anglais.
 

I’d go with cor anglais. There’s that kind of buzzy double-reed very focused timbre going on, and the clarinet family generally has more depth and “body” to the tone than the instrument in question.

Not impossible it’s one of the lower-pitched members of the clarinet family like the basset or tenor being played a bit higher than it’s comfortable with, but they’re rare instruments while the cor anglais is part of the standard orchestral instruments.

Of course it might be a synth....

rbecker
Probably an oboe or the lower, closely related english horn. It could be sax, but I doubt it in this context. I remember being told that - if you don't have an oboe in your ensemble - the oboe sound can be approximated by a clarinet and flute playing the line together with the flute up an octave.

Unique analysis! Interesting to see how mixtures of instrumental timbres can actually be equated to the raw sounds of other instruments. Just this past Sunday, I was visiting with a family friend where a few folks whipped out violin and tenor sax - with the violin playing over the tenor sax, I could've sworn there was a cello in the room. Very orchestral, warm beautiful sound as opposed to what is usually expected from the reediness of the saxophones.
 
Of course, the room sound gives its distinct flavor as well. Would've probably been a bit different if I were sitting right next to the T. sax player, for example.

My first thought was oboe.
 
Here is my favorite oboe piece :
 
https://www.youtube.com/watch?v=3P0TaRKQPbE
 
Cheers,
 
Bert

My first thought was Oboe as well, but that's because I have to confess i never think of English Horn in casual conversation unless it is brought up.

Oboe would be my first guess and then english horn.
 
One could check the score.

It must be english horn, this melody goes a fourth lower (F) than the lowest tone an oboe can play (Bb).

Brillyant!