Giuseppe di Stefano Mezza Voce Head Voice Compilation

Giuseppe di Stefano is my absolute favorite tenor for many reasons; but the biggest of all, and what I think makes his artistry so great, is his beautiful use of mezzo voce/head voice.

I've put in this compilation extracts from various recordings that demonstrate his soft singing. Does anyone know how he is able to produce this gorgeous, seamless sound?!

Extracts are:
O tu che in seno agl'angeli (1960)
Core n'grato (1964)
O dolce incanto... chiudo gli occhi (1944)
Una Furtiva Lagrima (1944)
E Lucevan le stelle (1950)
Salut demeure chaste et pure (1950)
Mi par d'udire ancora
Santa lucia luntana (1944)
Tu che a dio spiegasti l'ali (19??) (maybe the Callas/HVK recording?)
Lamento di Federico
Senerata Sincera (1959)
La strada del cuore (1954)
Cielo e mar (1950)

Видео Giuseppe di Stefano Mezza Voce Head Voice Compilation канала Posh Opera Singer