With awk (installed by default on POSIX systems including ubuntu):

awk -v X=2 '
  NR >= 2 && /^output-[0-9]{3}\.ts$/ && ++n % X == 1 {
    print "#Inserted line", i++
  }
  NR>1 {print last}
  {last = $0}
  END{if (NR) print last}' < file.in > file.out

That's for 2 lines above. To generalise to N lines above:

awk -v N=2 -v X=2 '
  BEGIN{N--}
  NR > N {
    if (/^output-[0-9]{3}\.ts$/ && ++n % X == 1)
      print "#Inserted line", i++
    print l[NR % N]
  }
  {l[NR % N] = $0}
  END{
    for (i = NR > N ? NR - N + 1 : 1; i <= NR; i++) print l[i % N]
  }'

With awk (installed by default on POSIX systems including ubuntu):

POSIXLY_CORRECT=1 awk -v X=2 '
  NR >= 2 && /^output-[0-9]{3}\.ts$/ && ++n % X == 1 {
    print "#Inserted line", i++
  }
  NR>1 {print last}
  {last = $0}
  END{if (NR) print last}' < file.in > file.out

That's for 2 lines above. To generalise to N lines above:

POSIXLY_CORRECT=1 awk -v N=2 -v X=2 '
  BEGIN{N--}
  NR > N {
    if (/^output-[0-9]{3}\.ts$/ && ++n % X == 1)
      print "#Inserted line", i++
    print l[NR % N]
  }
  {l[NR % N] = $0}
  END{
    for (i = NR > N ? NR - N + 1 : 1; i <= NR; i++) print l[i % N]
  }'

With awk (installed by default on POSIX systems including ubuntu):

awk -v X=2 '
  /^output-[0-9]{3}\.ts$/ && ++n % X == 1 {print "#Inserted line", i++}
  NR>1 {print last}
  {last = $0}
  END{if (NR) print last}' < file.in > file.out

With awk (installed by default on POSIX systems including ubuntu):

awk -v X=2 '
  NR >= 2 && /^output-[0-9]{3}\.ts$/ && ++n % X == 1 {
    print "#Inserted line", i++
  }
  NR>1 {print last}
  {last = $0}
  END{if (NR) print last}' < file.in > file.out

That's for 2 lines above. To generalise to N lines above:

awk -v N=2 -v X=2 '
  BEGIN{N--}
  NR > N {
    if (/^output-[0-9]{3}\.ts$/ && ++n % X == 1)
      print "#Inserted line", i++
    print l[NR % N]
  }
  {l[NR % N] = $0}
  END{
    for (i = NR > N ? NR - N + 1 : 1; i <= NR; i++) print l[i % N]
  }'